What Causes Algae in a Pool? 4 Failures, Always

Algae is always the same story: something let free chlorine fall, and something is feeding it. Four failures account for essentially every bloom, and the reason shocking keeps not working is that it addresses none of them.

The four failures

1. Free chlorine fell. The direct cause of almost every bloom. Hot weather, heavy use, strong sun and rain all consume or dilute it. Algae divides fast enough above 85°F that a pool can go from clear to visibly green overnight.

2. Cyanuric acid has climbed. This is the one that catches people out, because the chlorine test reads fine. CYA stabilises chlorine against UV, and stabilised tablets add more of it every time. As CYA rises, more chlorine is needed for the same sanitising effect — the common rule of thumb is free chlorine at about 7.5% of CYA. At 100 ppm CYA, a “normal” 3 ppm of chlorine is doing very little.

3. Circulation leaves dead spots. Algae appears first behind ladders, under steps, in corners and behind return fittings — anywhere water barely moves. The chlorine is in the pool and not getting there. Aiming returns to create a circular flow, and brushing those areas weekly, deals with most of it.

4. Something is feeding it. Phosphates from rain run-off, fertiliser drift, leaf litter and some pool chemicals. Phosphate does not cause a bloom in a properly chlorinated pool, but it makes one faster and more stubborn once chlorine slips.

Why shocking alone keeps failing

Shocking oxidises the existing bloom. It does not change the chlorine level you will hold next week, the CYA that is neutralising it, the dead spot behind the steps, or the phosphate load.

A pool that blooms, gets shocked, clears, and blooms again in two weeks is telling you the maintenance level is wrong — not that the shock was too small.

The sequence that actually resolves it:

1. Test CYA first. If it is above about 80 ppm, no amount of chlorine management will be efficient. Partial drain and refill is the only practical way down.
2. Correct pH to 7.2–7.6, before adjusting chlorine.
3. Shock, and brush every surface — especially behind ladders, steps and corners.
4. Run the filter continuously until clear, backwashing as pressure rises.
5. Then hold free chlorine at 2–4 ppm against a sensible CYA, and test twice weekly.

The patterns worth recognising

• Green two or three days after heavy rain — dilution plus phosphate wash-in. Test straight after a storm, not when the colour appears.
• Green only in corners and behind steps — circulation, not chemistry.
• Green despite “good” chlorine readings — CYA or pH.
• Yellow-brown dust on the shaded wall — mustard algae, which is chlorine-resistant and needs an elevated shock plus thorough brushing.
• Black spots in plaster — black algae, rooted into the surface. Brush with a stainless brush and treat the spots directly; it will not clear by chlorination alone.