Why pools turn green in the first place
Algae is always present in pool water. The only thing keeping it invisible is a free chlorine level high enough to kill it faster than it reproduces. When chlorine reaches zero (even for a day) a bloom starts, and in Illinois July heat it can go from faintly cloudy to fully green overnight.
The usual culprits are a stretch of hot weather that burned chlorine off faster than usual, a heavy rain that diluted the pool and dumped organics in, a stabilizer (cyanuric acid) level that is far too low to protect chlorine from sunlight, or a pump that quietly stopped circulating. Vacation weeks are the classic setup: nobody tests for ten days, and the pool is green when you get home.
The recovery sequence, in order
Order matters more than effort here. Shocking a pool with pH at 8.0 wastes most of the chlorine you just paid for, because high pH cripples its killing power. Work through these in sequence rather than doing everything at once.
- Day 1, Test and correct pH to about 7.2. This single step decides how well everything after it works.
- Day 1, Clean out baskets, skim the surface, and brush every wall, step, and corner. Brushing breaks the algae’s protective layer so chlorine can reach it.
- Day 1, Shock hard. A light dose on a green pool accomplishes nothing; see the next section for the amount.
- Days 1-5, Run the pump continuously. Filtration, not chemistry alone, is what physically removes dead algae.
- Days 2-4, Brush daily and re-test chlorine. Keep the level elevated until it stops falling overnight.
- Days 2-5, Clean or backwash the filter every time pressure rises 8-10 psi over its clean baseline. On a bad bloom this is several times.
- Day 3-5, Once the water turns cloudy grey-blue, you have won. Vacuum the dead algae to waste if your system allows it.
How much shock you actually need
This is where most homeowners fall short and end up fighting the same bloom for two weeks. Clearing algae takes shock chlorination. Driving free chlorine to roughly ten times the level of combined chloramines in the water, and holding it there. In practice, a visibly green pool typically needs three to four times a normal shock dose, added at dusk so sunlight does not burn it off before it works.
Buy more shock than you think you need. Running out halfway through is how a three-day recovery becomes a three-week one, because the algae rebounds every time chlorine dips. And skip the algaecide until the water is clear: most algaecides work as preventives, several foam badly at shock-level chlorine, and copper-based ones can stain a liner in a pool this far out of balance.
Reading the color: green, mustard, black, and cloudy
Not every discoloration is the same organism, and the tougher ones need more than a standard shock.
- Green, free-floating: ordinary green algae. Responds well to the sequence above.
- Yellow or mustard, clinging to shaded walls: mustard algae. Chlorine-resistant, needs a stronger and longer shock plus brushing, and it comes back if you miss any of it.
- Black or dark spots with roots in plaster: black algae. Rare in vinyl pools but stubborn. The heads must be scrubbed open before chemicals do anything.
- Cloudy but not green: usually a filtration or balance problem rather than algae. Check filter pressure, run time, and calcium hardness first.
- Green tint with clear water and no slime: often dissolved metals such as iron or copper oxidizing after a shock, not algae. Shocking harder makes metal staining worse, so confirm before dosing.
Preventing the next bloom
Nearly every green pool we recover shares one of three root causes, and all three are cheap to fix. Keep free chlorine between 1 and 3 ppm and test two to three times a week in peak season rather than weekly. Keep cyanuric acid in the 30-50 ppm range. With too little, summer sun destroys your chlorine within hours; with far too much, chlorine becomes sluggish and ineffective.
Third, run the pump long enough to turn the water over completely each day. A pump on a short timer schedule during a heat wave is a bloom waiting to happen. If you would rather not think about any of this, a weekly service plan exists precisely so a professional catches the drift before the water changes color.
When to stop and call someone
Call if you cannot see the bottom of the shallow end after 48 hours of correct treatment, if chlorine will not hold overnight no matter how much you add, if the bloom returns within days of clearing, or if the filter cannot keep up. Those symptoms usually mean something structural. A failing filter, a circulation dead zone, badly out-of-range stabilizer, or metals in the water, and no amount of shock fixes a mechanical problem.
Recovering a severe bloom also burns through a meaningful amount of chemicals. At some point the cost of guessing exceeds the cost of a service call that diagnoses the actual cause on the first visit.