Are You Ready?

The official start of summer may be Memorial Day weekend; however, the start of pool season can be as unpredictable as the weather. Warmer weather, longer days, and school breaks are just around the corner. That means more chemical consumption, increased activity, and more swimmers in the pool. It also means the threat of algae in the pool. Now is the time to get ready.

How Long Before a Pool Goes Bad?

As weather warms up and more bodies dive into the pool, sanitizer levels can get consumed rapidly. A pool that is exposed to increasing hours of sunlight can go through 1 part per million (ppm) of chlorine a day, even with cyanuric acid (CYA). Increased water temperature and swimmer load can lead to as much as 10 ppm of chlorine consumed within a week. So, what would happen if the chlorine added on Monday fizzles out by Thursday? Well it takes algae about 8 hours to double in population in untreated water. Within 3 days there can be millions of algae present. By the following Monday, the pool could be well on its way to becoming a swamp.

Prevention Tips

If it has been a while since the pool has been super-chlorinated, now it is a good time to do a chlorine disinfection shock treatment. This is accomplished by using enough chlorine to raise the free chlorine to 20 ppm. 12.5% sodium hypochlorite (liquid chlorine) is the best way to accomplish this. It takes 2 gallons of liquid chlorine in a 10,000-gallon pool to get 25 ppm of free chlorine. Because it’s in a liquid form there is an instant reaction to produce a powerful residual of free chlorine. Once the chlorine level has returned to below 5 ppm, test the pools for phosphate and do a phosphate removal treatment. Prepare to vacuum to waste after the phosphate treatment. Once phosphates have been reduced, a good broad-spectrum algaecide may be used.
Ideally, the free chlorine level should continue to be ramped up, so it surpasses any additional chlorine demand from environmental contaminants such as leaves and pollen. The best thing to determine at each pool is what the level of total chlorine is along with the level of free chlorine. Once these two tests are known, simply subtract the free chlorine test reading from the total chlorine reading, the amount of combined chlorine can be determined. The combined chlorine level will be the number that is left when the free chlorine is subtracted from the total chlorine.

Example:
Total Chlorine test = 3 ppm
Free chlorine test = 1 ppm

3 ppm – 1 ppm = 2 ppm

In this example the combined chlorine is 2 ppm and higher than the free chlorine. The goal is to add enough chlorine to ensure that the free chlorine residual will be higher than the combined. In this case a shock of 20 ppm of chlorine would be needed in order to deal with the combined chlorine and raise the free chlorine level. However, not all combined chlorine can be reduced by shocking. There are organic forms of combined chlorine that are resistant to chlorine. If high levels of combined chlorine do not lower after shocking then the pool may need to be drained and lowered a few feet, then replenished with fresh water.

Algaecides should be incorporated in situations where there is high use of the pool and difficulty in maintaining a free chlorine level that consistently remains higher than the combined chlorine.

Here is a simple program to prepare for Spring:

• Test all water chemistry parameters – especially cyanuric acid (CYA) and total dissolved solids (TDS). The TDS should be no higher than 1500 ppm over the TDS of the source water. If the pool is over 1500 ppm higher than the source water, the pool will need to be diluted to lower the TDS in order to bring it closer to the TDS of the source water.
• CYA should be no higher than 50 ppm. If trichlor tablets are used during the warmer pool season then the CYA will need to be monitored to ensure it does not rise excessively. High levels of CYA over 100 ppm can lead to decreased chlorine effectiveness, algae outbreaks, and the need to maintain higher levels of free chlorine. There are drain calculator apps that can give the amount in gallons to drain in order to reach a desired target. Just enter the gallons of the pool, current level TDS, and the TDS of the source water. The app will tell you how much water to drain and refresh to get the proper target level.
• Do a chlorine shock raising free chlorine to 20 ppm and try to hold it for at least four hours
• Test and treat for phosphates once the free chlorine level is below 5 ppm. Phosphate level should be no higher than 500 ppb. Below 200 ppb is recommended.
• Add a good broad-spectrum algaecide early in the season to get a jump on algae before the water heats up. A Polyquat or a copper based are best.
• This could be a good time to add borates as well. Borate level should be no more than 50 ppm. Borates act as a buffer to keep the pH from drifting up. They also are algae preventatives and serve to soften the water to reduce the potential for calcium carbonate scale. Boric acid is the best and most economical form to use. One 50 lbs. bag of boric acid will treat 20,000 gallons of pool water to near 50 ppm. Test strips are available for testing borate.
• One of the most effective algae preventatives is simply a good brushing of the pool. If the pool has been sitting dormant in the off season, it is extremely vital now to begin a weekly regiment of thoroughly brushing the pool's surface.

Drought Tips

In many regions—especially across the West and Southwest—drought conditions tend to intensify as summer wears on. Hot weather combined with lower available water supplies may lead some communities to declare drought restrictions. This could have an impact on swimming pools and how the pools are maintained. Here are some tips for pool service techs during declared times of drought:

• When draining is restricted consider what types of chemicals are going in and how much solid by-product is being left behind. For example tri-chlor tablets will increase the cyanuric acid CYA levels. The increase of CYA also leads to more chlorine needed to disinfect and oxidize. The higher CYA also is a part of total dissolved solids TDS which leads to decreased disinfection and more corrosive water. Calcium Hypochlorite will leave calcium behind and also increase hardness and the TDS leading to cloudy water and scale formation. Liquid chlorine leaves only sodium chloride as a by-product. The build up of sodium chloride does not decrease disinfection or cause cloudy water and scale.
• Evaporation is the largest natural loss of water during hot spells. Any measures that can be taken to minimize evaporation can help. Keep pools covered when not in use. If the pool does not have a cover use a chemical evaporation suppressant. Avoid overuse of waterfalls, fountains, or aerators during drought conditions.
• Cleaning and backwashing of filters can require large amounts of water. In the case of backwashing try to keep this to a minimum. Watching the site glass and turning the water off immediately when the water is clear. The pool can be treated to keep filters cleaner and more efficient during droughts by the regular use of enzymes and a natural based clarifier. Enzymes break down much of the organic oil-based contaminants that lead to filter clogging. A natural chitosan clarifier flocs tiny organic contaminants making backwashing more efficient at removal of the filter waste.
• If the pool needs to be topped up make sure not to overfill. For service professionals simply clipping your vehicle keys to the fill handle will ensure you won’t leave without turning off the water.

Learn more about water chemistry, testing, and regular maintenance with HASA.

Try our Pool Chemical Calculator and achieve perfectly balanced water today.

Author: Terry Arko

Terry Arko brings more than 40 years of experience in the recreational water industry and currently serves as Technical Content and Product Training Consultant for HASA Pool, makers of HASA Sani-Clor. His career spans service and repair, retail sales, chemical manufacturing, technical service, commercial sales, and product development—giving him a uniquely comprehensive perspective on water chemistry from both the field and the lab.

A respected educator and thought leader, Terry has written over 200 published articles on water chemistry and has been teaching chemistry-focused courses for more than 30 years. His ability to translate complex science into practical, real-world understanding has made him a trusted resource for pool professionals seeking safer, smarter, and more consistent water care.

Terry is a Certified Pool Operator (CPO) Instructor with the Pool & Hot Tub Alliance (PHTA) and serves on the Recreational Water Air Quality Committee (RWAQC). In 2025 he was presented the Pool Nation Lifetime Achievement Award. Terry is driven by a lifelong commitment to education, innovation, and advancing industry standards through knowledge.