Algae in Pool Cartridge Filters: Prevention & Cleaning

Your pool water looks crystal clear, but when you check the cartridge filter, you find green or yellow slime coating the pleats inside. That’s algae living inside your filter. Most pool owners don’t realize cartridge filters can harbor algae spores that survive the pool shock treatment, then multiply inside the cool, dark filter housing.

We have seen this problem before, and it’s more common than most people think. This guide covers how to identify filter algae, clean it out properly, prevent it from coming back, and decide whether your cartridge is worth saving or needs replacement.

Why Algae Grows Inside Cartridge Filters

Algae spores are everywhere in the Inland Empire. They blow in on the wind, come from landscape runoff (especially in neighborhoods with heavy irrigation), and thrive in warm water with insufficient chlorine residuals. When you shock your pool to kill algae, the chemicals kill algae in the water column. But some spores escape into the cartridge filter before the chlorine reaches them.

Inside the filter, conditions are perfect for algae to survive. The cartridge stays damp between filter cycles. The pleats create narrow channels where moisture lingers. Algae doesn’t need sunlight like it does in the pool itself. It feeds on residual organic matter trapped in the filter fibers.

If your pool is recovering from an algae bloom and the filter stays wet for days without running, that’s when you’ll find growth. We have seen green slime completely cover the inside of a cartridge within 48 hours after a heavy shock treatment.

The issue compounds because a dirty, algae-covered cartridge filters poorly. Algae reduces water flow. Your pool circulation slows. Water clarity drops again. And the spores trapped in the cartridge re-infect the pool water on the next filter cycle.

How to Identify Algae in Your Cartridge Filter

The easiest sign is visible growth. Open your filter tank and pull the cartridge. Look at the pleats. If you see green, yellow, brown, or black slime, that’s algae or mold. Sometimes the growth is light. Other times it’s heavy enough that you can see it from across the yard.

You might also notice:

Filter pressure rising faster than usual between backwashes or cleanings
Pool water that never quite clears, even after you shock
A musty or earthy smell when you open the filter housing (that’s the organic decay algae leaves behind)
Cloudy water that returns within a few days of shocking

If your cartridge filter is more than three to five years old and you find algae, this is a good time to consider replacement rather than deep cleaning. Older cartridges lose their filtration efficiency faster once they are compromised by algae growth.

Step-by-Step: Deep Clean Your Cartridge Filter

This is the fix our crew uses on service calls. It works for Hayward Star-Clear and Super-Star-Clear cartridges, Pentair Clean & Clear and Predator models, Jandy CL and CV cartridges, Polaris, and most other standard cartridge filters.

What You Will Need

Two five-gallon buckets or a large plastic tub
Trisodium phosphate (TSP) or a commercial cartridge cleaner like Jack’s Magic Filter Cleaner
Liquid bleach (5.25% sodium hypochlorite)
A soft brush or an old toothbrush
A hose with a spray nozzle
Work gloves
Eye protection
A clean, level surface to lay the cartridge while it dries
Four to six hours of time (most of it waiting, not active work)

Procedure

Step 1: Remove and Rinse the Cartridge (15 minutes)

Turn off the pump and filter system. Turn the multiport valve to “Closed” or “Off.” Open the filter tank drain plug at the bottom to release pressure. Wait 30 seconds for the tank to depressurize fully. Then unbolt the clamp ring or loosen the top clamp that holds the cartridge in the tank.

Pull the cartridge straight up and out. Set it on a level surface (a clean driveway works fine).

Use the hose and spray nozzle set to a medium-pressure spray (not a hard jet, which can damage the pleats). Spray the entire cartridge from top to bottom. Pay special attention to the area between the pleats. Spray from the outside in, and then from the inside out. You are rinsing away the loose algae, debris, and dirt.

Rinse until the water running off the cartridge is clear. This takes five to ten minutes.

Step 2: Soak in TSP Solution (12 to 24 hours)

Fill one bucket or tub with cool water. Add one pound of trisodium phosphate per five gallons of water, or follow the label on your cartridge cleaner. Stir until dissolved.

Submerge the cartridge completely. The TSP will break down the algae cell walls and loosen the slime layer. Let it soak for at least 12 hours. Overnight is ideal. If the water turns green or brown, the algae is coming off. That is the TSP working.

You can soak for up to 24 hours without risk. Many pool services soak cartridges overnight, pull them the next morning, and finish the job.

Step 3: Bleach Soak (10 to 30 minutes)

After the TSP soak, pull the cartridge out and rinse it thoroughly with fresh water again.

Fill the second bucket with water. Add one quart of liquid bleach per five gallons of water (a 1:5 ratio). This creates a mild bleach solution strong enough to kill remaining algae spores and mold but not so strong that it damages the filter media.

Submerge the cartridge in the bleach solution for 10 to 30 minutes. Agitate it gently a couple of times during the soak. If the algae is stubborn, stay closer to 30 minutes.

Do not leave the cartridge in bleach longer than 30 minutes. Prolonged bleach exposure breaks down the filter fibers.

Step 4: Rinse Thoroughly (20 minutes)

This step is critical. Any bleach residue left in the cartridge will get into your pool water when you run the filter again. Bleach residue will also cloud the water and burn eyes and skin.

Use the hose and spray nozzle again. Rinse the cartridge from every angle. Spray between the pleats multiple times. Pay attention to the end caps. Water should run clear, and you should not smell bleach.

Rinse for at least ten minutes. If you are not sure, rinse for 15.

Step 5: Dry the Cartridge (4 to 6 hours, or overnight)

Set the cartridge upright on a clean surface in a shaded area. Do not lay it on its side. Let it air dry completely before putting it back in the filter tank.

This is the hardest part for most people, because it is boring and takes time. But installing a wet cartridge traps moisture inside the filter housing, which promotes algae and mold regrowth. Wait for it to dry.

In the Inland Empire heat, a cartridge dries completely in four to six hours on a sunny day. If you are doing this in the evening, overnight drying is best.

Step 6: Reinstall and Resume Operation (10 minutes)

Once the cartridge is completely dry, carry it back to the filter tank. Insert it carefully, making sure the top and bottom are seated properly. Install the clamp ring or top clamp and tighten the bolts by hand until snug. Do not over-tighten; you will crack the end caps.

Close the drain plug. Turn the multiport valve back to “Filter.” Prime the pump if needed, and restart the system.

Let the filter run for two hours before checking the pressure gauge. Write down the clean filter pressure. This is your baseline. When the pressure rises eight to ten pounds above baseline, it is time to backwash (or pull the cartridge again next year).

Brand-Specific Guidance

Hayward Star-Clear and Super-Star-Clear Cartridges

Hayward cartridges are durable and handle aggressive cleaning well. The pleats are tightly spaced, which means algae can hide deep in the folds. When you deep clean a Hayward cartridge, spend extra time on the TSP soak (12 to 24 hours is worth it). The bleach soak at the standard 1:5 ratio works fine.

Hayward cartridges over five years old are good candidates for replacement if you find algae. The filter media thins with age, and algae growth is a sign of stress in the cartridge structure.

Pentair Clean & Clear and Predator Cartridges

Pentair cartridges are excellent filters but are more delicate than Hayward. When you rinse with the hose, use medium pressure, not high pressure. The pleats tear more easily.

The TSP soak works great on Pentair cartridges. For the bleach soak, use a slightly weaker solution (one quart bleach per six gallons of water) and limit the soak to 10 to 15 minutes. Pentair recommends not exceeding 20 minutes in bleach.

Pentair cartridges can last six to seven years if you stay on top of maintenance. If you find algae on a three-year-old Pentair cartridge, deep clean it. If you find algae on a six-year-old Pentair cartridge, replace it.

Jandy CL and CV Cartridges

Jandy cartridges are very common in Inland Empire pools and spas. They are tough and forgiving. The pleats are slightly wider than Hayward, so algae growth is usually easier to see and clean.

Jandy cartridges respond well to the standard TSP soak and bleach soak times (12 to 24 hours for TSP, 10 to 30 minutes for bleach). No special adjustments needed.

Jandy cartridges hold up well for six to seven years. If you have a Jandy cartridge and algae appears, deep clean it aggressively. The cartridge can handle it.

Cost and When to Replace vs. Clean

Cost of Deep Cleaning Your Own Cartridge

Supplies cost $10 to $20: trisodium phosphate or cartridge cleaner ($8 to $12), bleach ($2 to $5), and maybe a brush if you don’t have one ($3 to $5). Your time is four to six hours of mostly waiting. Total out-of-pocket: under $25.

Cost of Professional Cleaning

If you call the crew, we will pull the cartridge, deep clean it at our facility with industrial TSP and bleach solutions, pressure wash it, dry it, and reinstall it. Cost: $75 to $125 depending on the cartridge size and condition. Turnaround is usually one day.

Cost of Replacement

A new cartridge for a standard residential filter runs $80 to $200 depending on brand and size. Hayward and Pentair cartridges are usually on the higher end. Jandy and generic cartridges are on the lower end. Professional installation adds $50 to $75 in labor.

When to Replace Instead of Clean

Replace, not clean, if any of these apply:

The cartridge is six years old or older and shows algae. At this age, the filter media is thinning. Another season of heavy use will require replacement anyway.
You find cracks or tears in the pleats during the rinse step. Once the cartridge is physically damaged, deep cleaning will not restore it.
The algae growth is so heavy that you cannot remove it even after soaking and bleach. This usually indicates the cartridge has reached its service life.
You have already deep cleaned this cartridge twice in the last two years. A cartridge that needs this much attention is ready to retire.
Your filter pressure (after a thorough backwash or cleaning) stays higher than it should. A high-pressure baseline is a sign the cartridge is clogged beyond recovery.

The cost of a new cartridge is usually justified if it keeps your pool running trouble-free for another three to five years. The cost of labor and chemicals to deep clean a cartridge that fails again in a month is wasted money.

How to Prevent Algae in Your Cartridge Filter

Prevention is cheaper than cleaning. Here’s what we tell our customers.

Maintain Proper Chlorine Residuals

The single biggest factor in preventing algae is keeping your free chlorine level between two and four parts per million year-round. In summer, it should be on the higher end, closer to three or four ppm.

Algae spores cannot establish themselves in properly chlorinated water. The chlorine kills them before they reach the filter.

If your chlorine level drops below one ppm, algae can gain a foothold in 12 to 24 hours. Test your chlorine level at least twice a week. More often is better.

Run Your Pump Daily, Especially in Summer

During the hot months (May through September in the Inland Empire), run your pump and filter every single day. Run it for at least eight hours, preferably 12. Some pools benefit from 24/7 circulation during the peak heat season.

Why? Because constantly circulating water passes algae spores through the filter where the chlorine residual can kill them. Stagnant water in a pool left unfiltered for two days during hot weather is an algae farm.

Brush the Pool Walls and Floor During Hot Months

During summer, brush the pool walls and floor at least three times a week. Algae loves to settle on surfaces where water movement is slow. Brushing disturbs those settlements and pushes algae into the water column where the chlorine and filter can deal with it.

Shock Your Pool After Heavy Use

After a weekend pool party, a day camp, or any time you have heavy bather load, shock your pool that evening. The bathers introduce sweat, body oils, and organic waste that consume chlorine. The shock restores the residual.

If You Have an Algae Bloom, Shock Twice

If your pool does turn green or cloudy, shock it that day. Wait 24 hours (do not swim). Then shock again. The double shock drives the algae bloom to completion and kills the spores that survived the first treatment.

After the double shock, run the filter continuously for two to three days. Backwash (or pull the cartridge and rinse) every four to six hours during the heavy recovery period. This keeps dead algae cells from settling in the filter and composting into food for new growth.

Keep Your Filter Clean

Backwash or rinse your cartridge every one to two weeks during the swimming season. Do it more often if you are brushing the pool regularly or if you have high bather load. A clean filter circulates water efficiently. A clogged filter creates dead spots where algae thrives.

Watch for Phosphates

Phosphates feed algae. They come from fertilizer runoff, certain cleaning products, and decomposing organic matter. In the Inland Empire, phosphates spike during irrigation season when yard chemicals get washed into pools.

If your pool has a recurring algae problem even with good chlorine levels, test your phosphate level. If it is above 500 ppb (parts per billion), use a phosphate remover product once per season. It is not necessary every week, but once in spring or early summer can prevent an entire season of algae headaches.

Frequently Asked Questions

What does algae in a pool filter look like?

Green, yellow, brown, or black slime coating the inside surfaces of the cartridge pleats. Sometimes it looks like a fuzzy film. If the algae is heavy, the pleats turn dark and slimy. Light algae growth looks like a thin coating you can only see when the light hits it at an angle. Pull the cartridge into bright sunlight and look closely. If something is there that wasn’t there last month, that’s algae.

Can I clean my cartridge filter with vinegar instead of TSP?

Vinegar is weak and will not break down algae as effectively as trisodium phosphate. If you want to use vinegar, soak the cartridge for 24 to 48 hours. It’s not as good as TSP, but it’s better than nothing. TSP is inexpensive and works reliably, so we recommend spending the few dollars on TSP instead.

How often should I deep clean my cartridge filter?

If your pool and filter are well-maintained, you should never need to deep clean a cartridge. A normal backwash or rinse between the pleats every one to two weeks is enough. If you find algae in your cartridge once, deep clean it. After that, focus on prevention (chlorine levels, daily circulation, brushing). If you are deep cleaning a cartridge more than once per year, there is a bigger problem with your pool maintenance routine or your pool’s location.

Can I put my cartridge in the dishwasher?

No. The hot water can warp the cartridge, and the cartridge material can fragment and damage the dishwasher. Hand clean only.

Is it safe to swim while the cartridge is drying?

Yes, as long as you have another way to circulate the water. Some people have a spare cartridge they keep on hand for this reason. If you only have one cartridge, you can use the pool while the cartridge dries if you run it through a canister filter or sand filter temporarily. Do not leave the pool unfiltered for more than four hours during summer.

What if the cartridge has visible cracks or tears?

If the pleats have cracks or tears, replacement is the only option. Do not try to patch or glue a damaged cartridge. The water will find the weak spots and bypass the filter media entirely.

How long does it take to feel confident deep cleaning a cartridge myself?

The first time takes four to six hours because you are being cautious and learning. The second time takes two to three hours because you know the steps. By the third time, you can do it in 90 minutes. Many homeowners in the Inland Empire deep clean their own cartridges once a year and save hundreds of dollars in service calls.

When to Call a Professional

If any of these apply, call the crew:

You find algae in your cartridge and you are not comfortable pulling the filter tank apart yourself.
You suspect a crack or tear in the cartridge, but you are not sure.
Your filter pressure stays high even after a thorough cleaning, or it rises very quickly after backwashing.
You have cleaned the cartridge three times in the last year and algae keeps coming back. This usually means there is an underlying circulation or chemical issue we need to diagnose.
You want a professional to deep clean the cartridge using industrial-grade chemicals and equipment.

We offer free estimates on cartridge filter cleaning and replacement. We also stock replacement cartridges for Hayward, Pentair, Jandy, Polaris, and most other common brands.

Take Action Now

Algae in your cartridge filter is fixable. If you caught the problem early, a deep clean will restore your cartridge to like-new condition. If the cartridge is already old, replacement is a one-day job that will improve your water clarity and circulation immediately.

Do not ignore filter algae. The longer it sits, the harder it is to clean. The problem spreads to your pool water next.

Free estimate? Call us at (909) 330-4730. Same-week service available. We have seen this before, and we can fix it.