How to Kill Pool Algae
Algae is not one problem - it is four, and each one dies at a different chlorine level. Identify yours below, hit it with the right shock level, and brush like you mean it.
Step 1: identify your algae
- Green algae - the classic. Water turns hazy green, walls slimy. Easiest to kill. Shock target: about 10 ppm higher than normal FC (see calculator).
- Mustard (yellow) algae - powdery yellow-brown patches, usually in shady corners. Comes back if you under-shock. Shock target: 15-20 ppm FC, and it survives on pool toys and swimsuits - wash everything.
- Black algae - black or blue-green spots with roots anchored into plaster. The hardest kill. Shock target: 25-30 ppm FC plus aggressive daily brushing to break the protective head.
- Pink "algae" - actually bacteria (pink slime). Slime rings at fittings and ladders. Treat like mustard algae with 15-20 ppm FC.
Step 2: calculate your shock dose
Step 3: the kill process (do it in this order)
- 1. Brush everything - walls, floor, steps, ladders. Brushing rips algae off surfaces so chlorine can reach it. Skip this and you are just feeding it.
- 2. Clean or backwash the filter 24 hours after shocking, and again every day until the water clears. Dead algae clogs filters fast.
- 3. Run the pump 24/7 until the water is clear. No pump, no kill.
- 4. Re-dose to target every evening until the water is blue and FC holds overnight. If FC drops more than half overnight with no algae visible, something is still living - keep going.
- 5. Vacuum to waste (not through the filter) once dead algae settles. You will do this more than once. That is normal.
Why the algae came back - the real fix
- Free chlorine too low for your CYA. Chlorine strength depends on the ratio of FC to CYA. A pool with 50 ppm CYA needs roughly 4-6 ppm FC sitting in it to resist algae; the old advice "2-4 ppm is fine" assumes zero CYA.
- Poor circulation. Dead spots in the corners grow algae first. Angle your returns to spin the water and brush weekly even when the pool looks perfect.
- Phosphates and debris. Leaves and pollen feed algae. Skim daily, empty baskets, and keep the CYA in range so sunlight cannot nuke your chlorine.
Frequently asked questions
How much bleach does it take to kill green algae?
For a typical 15,000 gallon pool at 1 ppm FC, reaching a 15 ppm shock level takes about 116 fl oz (roughly 3.6 quarts) of 6% bleach. Brush first, dose in the evening, and re-dose daily until it holds.
Can I swim after shocking for algae?
Not until free chlorine drops back below about 10 ppm and the water is clear. Depending on dose and sunlight, that is usually 1-2 days. Test, do not guess.
Is algaecide needed?
Usually no. Chlorine at the right level kills every common algae. Algaecides help prevent regrowth in marginal cases (especially quats for mustard algae) but they do not replace shocking - and copper-based algaecides can stain plaster.
My pool is a swamp. Is it even saveable?
Usually yes - drain rarely helps and can float an inground pool out of the ground. Brush, shocking to 20+ ppm, filter cleaning every day, and patience for 3-7 days clears most swamps for under $100 in bleach.