You lifted the cover and it is green. First decide, then treat.

The decision nobody makes deliberately

Most people default to treating, because draining feels wasteful and because the pool supplies aisle is organized around products rather than decisions. On an inground pool that default is right — replacing 20,000 gallons is a serious expense.

On a seasonal above-ground pool it is not. At the EPA WaterSense national average of $6.06 per 1,000 gallons, a full refill of a 3,955-gallon pool is about $24 and a 1,718-gallon pool about $10. Against a heavy shock, several cartridges, an algaecide, days of continuous pump running and two weeks of not swimming, that is frequently the cheaper option in both money and time.

If you drain

It is not simply pulling the plug. Three things matter.

Where the water goes. Several thousand gallons arriving at one spot saturates ground, can undermine a path and in the wrong place finds a building. Drain slowly, move the outflow around, and direct it away from the house, the boundary and the pad the pool stands on. Local rules on discharging pool water vary and we cannot look yours up.

Never fully drain a pool that is still standing without checking. Some above-ground pools rely on the water for stability and some liners will shift or shrink once unweighted. Check your manual before emptying a pool you intend to keep up, and if in doubt leave a few inches and clean around it.

Clean before refilling. Wipe the liner while the walls are still supported if you can, and get the sludge out rather than letting fresh water sit on it. Then refill and treat it as a new fill: alkalinity first, then pH into the CDC’s published 7.2 to 7.8 range, then chlorine, with a dispenser so the level holds. Our spring shopping list is what you need on the shelf.

If you treat: the sequence

1. 1

   Get the debris out first

   Leaves, twigs and anything that came in over winter. Chlorine spent on organic debris is chlorine not spent on the algae, and this is the step that most often gets skipped in the rush to dose.
2. 2

   Fit a new cartridge before you start

   Not last year’s, rinsed. The filter is about to do the hardest work it will do all season, catching everything the shock kills. Starting on tired media turns hours of filtering into days — our cartridge guide covers it.
3. 3

   Test, then correct alkalinity and pH

   Chlorine works poorly outside the CDC’s published pH range of 7.2 to 7.8, so shocking before correcting is wasting the dose. Total alkalinity first, because it is what holds pH still. Our pH and alkalinity guide is the detail.
4. 4

   Brush every surface

   Walls, water line, floor, and specifically the shaded and low-circulation areas. Brushing exposes the algae to the chlorine; shocking an unbrushed pool treats the water rather than the problem.
5. 5

   Shock heavily, at dusk, pump running

   Unstabilized calcium hypochlorite hits hard and adds no cyanuric acid to a pool that may already have plenty. Dusk, because sunlight destroys unstabilized chlorine quickly. Half a dose achieves close to nothing here — our shock guide covers the amounts.
6. 6

   Filter continuously, and keep testing

   Not on the normal schedule — without a break, until the water clears. Re-shock if chlorine falls back to nothing overnight, which on a heavy bloom it will for the first day or two.
7. 7

   Vacuum the dead algae, then change the cartridge again

   Dead algae settles as a fine layer. Vacuum it slowly so you are not stirring it back into suspension, and expect the cartridge to be finished afterwards. Our manual tools page covers doing it properly.
8. 8

   Then a preventive algaecide

   Polyquat, at the label rate, once the water is clear. It is a preventive rather than a cure, so dosing it into an active bloom is largely wasted — this is the last step, not the first.

Check cyanuric acid before you spend a fortnight

One reading explains most failed recoveries. If stabilizer built up through last season and the pool was never diluted, your chlorine is bound and sluggish — it will read respectably and act poorly, which is exactly the scenario our cyanuric acid guide describes.

Treating a heavy bloom in a pool with high stabilizer is fighting with one hand tied. If the reading is high, a partial drain is not a detour, it is the fastest route — and on this size of pool it costs a few dollars. A seven-way strip gives you the number in fifteen seconds; five-way strips omit it entirely.

Why it happened, so it does not happen again

A green opening is almost always an autumn story rather than a spring one. The usual causes, in order of frequency: the pool was closed while the water was still warm and stayed biologically active under the cover; debris was left in it to decompose all winter; or the cover failed and let light and leaves in.

All three are covered in our winterizing guide, and the cost of a bad close puts numbers on exactly this morning. The single highest-value change is waiting for consistently cold water before closing rather than closing on the first cold week.

When to stop and just refill

Give yourself a limit before you start. If after two full days of continuous filtering with a fresh cartridge and correct chemistry you still cannot see the floor, the water is not worth the third day. Drain it, clean it, refill for the price of a takeaway, and start the season a fortnight earlier than you otherwise would have.

There is no prize for saving the water, and our cost-to-fill guide has the arithmetic for your own volume if you want to make that decision with a number rather than a feeling.