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Your Complete Pool Opening Guide
Plan ahead: Pool opening can take anywhere from a few days up to a full week depending on water condition. Start at least one week before you plan to swim.
You pull back the pool cover and the water is green. No need to panic. The fix is straightforward: a step-by-step chemical treatment plan. This guide covers exactly what to buy, what order to apply it, and why each step matters so you can get from green to blue as fast as possible.
Pool Opening Shopping List
- Cal hypo Pool Shock: Kills algae fast without causing chlorine lock. Apply in the evening with the pump running. Do not use in a floater or feeder.
- Algaecide: Apply 24 hours after shocking to stop algae from coming back. Add directly to pool water.
- Chlorinating Tablets: Use in a floater, feeder, or skimmer for ongoing weekly sanitizing. Never throw directly into the pool.
- Test Strips or Test Kit: Always test before adding any balancers.
- Balancers: Only add what your test results call for. Adjust in this order: alkalinity first, then pH, then calcium hardness.
- Flocculant (optional): Turn off your filter, add, and let dead algae drop to the bottom for vacuuming to waste. Requires the ability to vacuum to waste.
- Clarifier (optional): For blue-but-hazy water after vacuuming. Dilute in a bucket of pool water before adding.
- Metal, Stain & Scale Control (optional): Recommended if you have hard water or metal-heavy source water.
Step 1: Kill the Algae
Before anything else, the algae that appeared during the off-season needs to go. Skipping or rushing this step means everything you add afterward will be fighting an uphill battle. Work through the steps below in order.
Shock the Pool (Evening Only)
Shocking is the fastest way to spike chlorine levels high enough to wipe out algae. Always shock in the evening. Sunlight rapidly depletes free chlorine, so nighttime application gives the product time to work. Before shocking, confirm pH is between 7.2 and 7.8 and the pump is running. Broadcast granules evenly over the deepest part of the pool. If any granules settle on the pool floor, brush them immediately to disperse. Undissolved granules can bleach or damage pool surfaces. Do not enter the pool until free available chlorine (FAC) returns to 1-4 ppm.
Why cal hypo specifically? Always use a calcium hypochlorite (cal hypo) shock for pool opening, not a stabilized shock (trichlor or dichlor). Stabilized shocks add cyanuric acid (CYA) each time you use them. CYA never burns off the way chlorine does, so it accumulates over the season and eventually causes chlorine lock, where chlorine stops working effectively. The only fix at that point is a partial drain and refill. Cal hypo delivers a powerful chlorine boost with no CYA added. Never use shock in a floater or feeder.
Check the water the following morning. If algae is still visible, shock again the next evening. Do not move on to the next step until all algae is visibly dead.
HTH™ Pool Care Shock Ultra: All-in-One Cal Hypo Pool Shock
The go-to choice for pool opening. This all-in-one cal hypo formula boosts chlorine fast, fights bacteria and algae, helps reduce eye and skin irritation, prevents stain and scale, and improves filtration, all without adding CYA. Safe for vinyl liners and all pool types including saltwater systems. Expect brilliantly clear water within 24 hours. Apply in the evening with the pump running. Do not use in a floater or feeder.
HTH™ Pool Care Shock Advanced: 4-in-1 Pool Shock for Algae
A powerful 4-in-1 cal hypo formula that kills bacteria, prevents algae, reduces chlorine odor, and restores water clarity, all without adding CYA. Clears cloudy water in 24 hours. A strong choice when pool water carries a heavy chemical smell or significant contamination alongside the algae. Apply in the evening with pump running. Do not use in a floater or feeder.
HTH™ Pool Care Green to Blue Advanced: 2-Step Pool Shock and Flocculant
Built specifically for severe green pool recovery. Step 1 (Advanced I) is a cal hypo shock that kills algae and bacteria without adding CYA. Step 2 (Advanced II) is a flocculant that binds dead algae and fine particles, sinking them to the pool floor for vacuuming directly to waste. When used as directed with the proper equipment, this 2-step system can restore clear blue water in as little as 24 hours. Safe for vinyl-lined pools. Severe algae blooms may require more than one shock treatment. Do not use in a floater or feeder.
Equipment note: This system requires specific equipment and will NOT work without it. You must have a filter system with a multiport valve that includes Recirculate (or Bypass) and Waste settings, plus a manual pool vacuum that connects directly to the filter system. Robotic cleaners and vacuums that collect debris in mesh bags will not work with this product.
Remove Dead Algae
Once shock has done its job, the pool will be full of dead algae. It does not disappear on its own and needs to be physically removed. If dead algae is floating and hard to vacuum, use a flocculant first.
How flocculant works: Turn off the filter (not just the pump) to allow the flocculant to settle all debris to the bottom undisturbed. Once debris has settled to the pool floor (typically overnight), vacuum to waste so it exits the system entirely rather than recirculating back into the pool. Next, run the system in the normal "Filter" position.
HTH™ Pool Care Drop Out™ Flocculant
This pool flocculant binds together fine particles, dirt, and dead algae and drops them to the pool floor for vacuum-to-waste removal. The dual-action formula also removes phosphates that feed algae growth. Safe for all pool types and pH levels. Turn the filter OFF after adding to let debris settle overnight, then vacuum to waste.
Equipment note: Requires a manual pool vacuum connected directly to the filter system and a multiport valve set to Waste. Robotic or bag-style vacuums will not remove settled debris correctly.
Polish Out Remaining Cloudiness with a Clarifier
After vacuuming, turn the filter back on and run it continuously. If the water is blue but still hazy, the culprit is particles too small for the filter to catch on its own. A clarifier combines these tiny particles into larger clumps the filter can trap and remove. This product is highly concentrated, so always dilute it in a bucket of pool water before adding to the pool. Pour most of the diluted solution evenly around the pool perimeter, then add the remainder directly to the skimmer. Run the filter for at least 8 hours after application.
HTH™ Pool Care Clarifier Advanced
This fast-acting, dual-action clarifier clears dull, hazy, or cloudy pool water by clumping fine particles and dead algae so the filter can remove them. It does not affect pH balance. Highly concentrated, so always dilute in a bucket of pool water first, then pour most around the pool perimeter and the remainder into the skimmer. Run filter for at least 8 hours after application. Vacuum as needed to remove any settled debris.
Equipment note: Run the filter continuously for at least 8 hours after application. Clean or backwash the filter after treatment, as it will collect a significant load of trapped debris.
Step 2: Prevent Algae from Coming Back
With the algae gone, the next priority is keeping it that way. Algae spores are always present in pool water and only need the right conditions (low chlorine, imbalanced pH, warm water) to bloom again. This step establishes the ongoing chemical defenses that stop those conditions before algae gets a foothold.
Start Your Chlorine Routine Right Away
As soon as all algae is dead and removed, get the chlorine routine going. Chlorine tablets placed in a floater, feeder, or skimmer will slowly dissolve and maintain a consistent residual chlorine level between shock treatments. Never throw tablets directly into the pool as they can damage or bleach pool surfaces. Run the pump a minimum of eight hours daily. Replace tablets weekly, or as often as needed to keep chlorine between 1-4 ppm.
HTH™ Pool Care 3" Chlorine Tabs Advanced: Algae Protection
Multi-functional, slow-dissolving stabilized chlorine tablets that sanitize, clarify, and fight algae for up to one week per tablet. One tablet treats up to 10,000 gallons. Use in a floater, automatic feeder, or skimmer basket. Never throw directly into the pool. Replace weekly or as needed to maintain 1-4 ppm free available chlorine. Run pump a minimum of 8 hours daily.
Equipment note: Use with a floating dispenser designed for this product, an automatic chlorine feeder, or in the skimmer basket with pump running at least 8 hours daily.
HTH™ Pool Care 3" Chlorine Tabs Ultra: All-in-One
An all-in-one stabilized chlorine tablet that fights algae, sanitizes, clarifies, softens pool water, and helps protect pool surfaces. One tablet treats 10,000 gallons. Refresh weekly or as needed to keep chlorine at 1-4 ppm. Never throw directly into the pool.
Equipment note: Use in a floating dispenser, automatic feeder, or skimmer basket. Never place directly in pool water.
HTH™ Pool Care 3" Chlor Tabs: Individually Wrapped
Dependable slow-dissolving stabilized chlorine tablets, individually wrapped to reduce odor and dust for neater handling. These tablets sanitize, fight algae and help maintain crystal clear water. One tablet per 10,000 gallons. Replace weekly or as needed to maintain 1-4 ppm. Never throw directly into the pool.
Equipment note: Use in a floating dispenser designed for this product, an automatic feeder, or in the skimmer basket with pump running at least 8 hours daily.
HTH™ Brilliance Cal Hypo Chlorine Tablets: No CYA
For pool owners concerned about CYA buildup from stabilized tablets over the season, these calcium hypochlorite tablets sanitize, kill bacteria, prevent algae, and maintain brilliant clarity without adding any cyanuric acid nor cause “chlorine lock”. One tablet per 10,000 gallons. Replace weekly or as needed to maintain 1-4 ppm. Run pump a minimum of 8 hours daily. Never throw directly into the pool.
Equipment note: Use in a floater specifically designed for calcium hypochlorite tablets, an empty skimmer basket, or an HTH Skimmer Basket Insert. Do not use in a feeder designed for trichlor tablets.
Dispenser options for your chlorine tablets:
HTH™ Pool Care Chlorine Floater: Slow Release
Pre-loaded with 3" Chlor Tabs, simply follow the dosing instructions on the label, drop it into the pool, and let it float. The floater will tip on its side when the tablets have dissolved completely. Provides up to 4 weeks of low-maintenance sanitizing when used as directed.
Equipment note: Float directly in the pool. Remove from the water when swimmers are present. Do not allow it to rest against pool walls, steps, or liners, as contact can cause bleaching or damage.
HTH™ Pool Care Skimmer Basket Insert: No-Touch Tablet Holder
Holds 3-inch chlorinating tablets inside your existing skimmer basket so tablets can be replaced without direct contact. Also helps with debris removal at the same time. A convenient option for consistent chlorination through the skimmer.
Equipment note: Attaches to your existing skimmer basket. Compatible with standard in-ground and above-ground skimmers.
Apply Algaecide: 24 Hours After Shock
Wait a full 24 hours after shocking before applying algaecide. Adding it too soon means the elevated chlorine level will break it down before it can do its job. After 24 hours, add algaecide directly to the pool water. For best results, apply algaecide as part of regular maintenance, before any signs of algae appear, not after.
HTH™ Pool Care Algae Guard Ultra: Extra Strength
HTH’s most powerful algaecide. This non-foaming, non-staining professional-grade formula kills and prevents tough green, black, and mustard algae and protects against regrowth for up to 30 days. Add directly to pool water and swim right away. Safe for all pool types including vinyl-lined and saltwater systems. For best results, use every month as part of your maintenance routine.
HTH™ Pool Care Algae Guard Advanced: All Algae Types
Professional-strength, fast-acting, non-foaming algaecide that destroys and prevents green, black, and mustard algae. Add directly to pool water and swim immediately. Safe for all pool types including vinyl-lined and saltwater systems. A strong choice for all-season weekly protection.
HTH™ Pool Care Algae Guard 10: Green Algae Control
A non-foaming, fast-acting algaecide formulated to kill and prevent green algae. Add directly to pool water and wait 15 minutes before swimming. Safe for all pool types including vinyl-lined and saltwater systems. Use as part of a weekly maintenance routine for best results.
Step 3: Test and Balance Your Water for Swimmer Comfort
Once algae is gone and the chlorine and algaecide routine is running, it’s time to focus on water chemistry. Balanced water keeps swimmers comfortable, protects equipment, and extends the life of pool surfaces. Always adjust one product at a time, in this order: alkalinity, then pH, then calcium hardness. Never mix balancers together before adding them to the pool.
Test Your Water First, Every Time
Before adding any balancers, test the water. Your test results tell you exactly which chemicals you need and how much. You may not need all the balancers listed here, so only add what the results call for.
How to use the HTH™ Test to Swim™ App: Dip one test strip into the pool water for 2 seconds, wrist-deep and away from jets or return lines. Remove with pads facing up and shake once. Do not shake off excess water. Hold the strip level for 20 seconds, then snap a photo in the app for personalized results and dosing instructions. You can also text a photo to 484-123 for the same guidance by text.
HTH™ Pool Care 6-Way Test Strips
Measures 6 key water chemistry parameters in seconds: free chlorine, bromine, pH, total alkalinity, calcium hardness, and cyanuric acid (CYA). Use with the HTH™ Test to Swim™ app for personalized treatment instructions, or compare to the color chart on the bottle. Works with all pool types including chlorine, bromine, and saltwater systems.
A drop-based liquid test kit for pool owners who prefer a more hands-on, precise reading. Measures the same 6 parameters as the test strips. A solid choice for double-checking results or for those who prefer not to use strips.
Apply Balancers in Order: Alkalinity, then pH, then Calcium Hardness
Adjust one chemical at a time and allow each product to fully dissolve before adding the next. Never mix pool chemicals together. Wear protective gloves and eyewear when handling any balancer.
HTH™ Pool Care Alkalinity Up: Total Alkalinity Increaser
Raises total alkalinity and prevents pH from bouncing out of range. Low alkalinity leads to unstable pH, which causes eye and skin irritation, corrodes equipment, and damages pool surfaces. Always adjust alkalinity before touching pH. Safe for all pool types including saltwater and vinyl-lined pools. Target range: 60-120 ppm.
HTH™ Pool Care pH Up: pH Increaser
Raises low pH to improve chlorine efficiency and swimmer comfort. When pH drops below 7.2, water turns corrosive, which irritates eyes and skin and damages equipment and surfaces. Adjust after alkalinity is in range. Safe for all pool types. Target range: 7.2-7.6.
HTH™ Pool Care pH Down: pH Decreaser
Lowers high pH to improve chlorine performance and prevent scale and staining. High pH reduces chlorine effectiveness, clouds the water, and leads to mineral scale buildup on surfaces and equipment. This product also lowers alkalinity, so always adjust alkalinity first. Safe for all pool types including vinyl-lined and saltwater systems. Target range: 7.2-7.6.
HTH™ Pool Care Calcium Up: Calcium Hardness Increaser
Raises calcium hardness to protect pool surfaces and equipment. Water low in calcium becomes corrosive and will leach minerals from plaster, vinyl, and equipment. Broadcast evenly over pool surface with pump running and brush any settled material immediately to ensure it dissolves. If a large dose is needed (more than 50 ppm increase), add one-third of the total dose at a time and wait 4-6 hours between additions. Target range: 200 ppm and above.
HTH™ Pool Care Chlorine Stabilizer: Cyanuric Acid (CYA)
Protects chlorine from being rapidly destroyed by UV sunlight. Without adequate stabilizer, the sun can wipe out most free chlorine in just a few hours. Apply directly to the skimmer if CYA tests below 20 ppm. Target range: 20-25 ppm. If you use stabilized chlorine tablets all season, CYA may already be in range, so always test before adding.
Equipment note: Apply directly to the skimmer with the pump running.
Address Hard Water and Metals If Needed
If your water source has high metal content (iron, copper, or manganese) or high hardness, you may see water that appears green, brown, or black for reasons unrelated to algae. Staining on pool surfaces and equipment is another common sign. If this applies to your pool, add HTH™ Metal, Stain and Scale Control at opening and again as part of your ongoing maintenance routine.
HTH™ Pool Care Metal, Stain and Scale Control
Prevents dissolved metals from staining pool walls, floors, and fixtures and guards against mineral scale buildup on filters, heaters, and pumps. Also effectively lifts new metal and copper stains. Initial treatment: 10 oz. per 10,000 gallons, added to the deepest part of the pool. Follow with regular maintenance dosing according to on-pack instructions.
Step 4: Keep It Clean All Season with a Twice-Weekly Routine
The heavy work is done. Now consistency is everything. A regular maintenance schedule is far easier and far less expensive than treating another algae outbreak mid-summer. Pick two days per week and protect them.
Your Twice-Weekly Pool Care Checklist
- Test the water using test strips or a test kit
- Add balancers as indicated by test results, one at a time and in order: alkalinity, then pH, then calcium
- Replace chlorinating tablet once per week, or as needed to maintain 1-4 ppm free available chlorine
- Shock the pool in the evening with the pump running. Do not reenter the pool until FAC returns to 1-4 ppm
- Apply algaecide 24 hours after shocking, added directly to pool water
Tuesday and Thursday work well for most pool owners who want clear water going into the weekend. Sunday and Tuesday help recover the pool after heavy weekend use. The specific days matter less than sticking to them.
Keep Your Filter Clean, Too
A clean, functioning filter is the backbone of clear water between treatments. All the shock and clarifier in the world will underperform if the filter is clogged with debris. Clean it at the start of the season and 2-3 times throughout.
HTH™ Pool Care Filter Cleaner: For Cartridge, Sand and DE Filters
Professional-strength universal filter cleaner that removes oils, minerals, and debris from sand, D.E., and cartridge filters to restore full filtration efficiency. Compatible with all pool types including saltwater systems. Use as part of a regular maintenance routine. Refer to on-pack instructions for your specific filter type. Plan to replace cartridges every 2-3 years and the sand every 3-5 years.
HTH™ Pool Care D.E. Filter Aid: DE Pool Filter Powder
If you have a diatomaceous earth (DE) filter, recharge it with fresh DE powder after each backwash. Mix the powder into a slurry first by combining 1 part powder with approximately 10 parts water until the mixture has a milky appearance. With the pump running, slowly pour the slurry into the skimmer nearest the filter. Catches tiny debris that causes cloudy or hazy water and keeps your DE filter performing at full capacity. Use approximately 1.5 lb per 10 sq ft of filter surface area and refer to your filter manufacturer’s guide for the exact surface area.
Equipment note: Always slurry the DE powder in water before adding through the skimmer. Do not pour dry powder directly into the skimmer. Follow the filter manufacturer’s instructions for the correct amount for your specific filter.
Still have questions? Call the HTH™ helpline at 1-866-HTH-POOL or chat live with an expert at hthpools.com. Available Monday through Friday, 9 a.m. to 7 p.m. ET. You’re All Clear with HTH™.
