Pool Closing Chemicals

Which size closing kit do I need?

Go by volume. The figure printed on a kit counts water, not pool dimensions, which trips up more people than it should. Ours run from 10,000 up to 35,000. If you do not know your volume, length times width times average depth times 7.5 gets you close on a rectangle, and average depth means halfway between the shallow end and the deep end rather than the deepest point. Round up to the next kit if you land between two. Under dosing is the one mistake that shows, because the algaecide runs out in February and you open green.

Do I still need a kit if I shock the pool first?

They are doing different jobs. Shock clears what is in the water now and is gone within days; the kit is what keeps working once the pump stops and the cover goes on. A winter algaecide is built to stay active in cold water for months, which ordinary algaecide is not, and a metal control control stops metal coming out of solution in water that is going to sit still until spring. Shocking is a good idea before you close. It is not a substitute.

What is in a closing kit?

It varies by maker, which is worth knowing before you compare two on price. Most combine a winter algaecide, an oxidiser or shock, and something that holds metal in solution, and some add a slow release sanitiser. The floating ones, the Chlorine Sleeper and the WinterPill, are a dispenser rather than a set of bottles: you drop one in and it meters itself out over the season. Read the box for what it actually contains rather than assuming a kit at one size matches a kit at another.

Does my pool need a metal or stain control?

Only if your water carries metal, and most people find out the expensive way. Well water and some municipal supplies carry iron or copper, and copper also arrives from a heater or an ionizer. In moving, filtered water it stays in solution and you never see it. In cold, still water under a cover for five months it comes out and lands on the walls, and what it leaves behind is a stain you are treating in May rather than a haze you can filter. If you have ever had a green or rust coloured mark appear at opening, you need it.

Can I use what is left over next season?

Some of it, and the liquid is the part to watch. An unopened kit stored somewhere that does not freeze is generally fine for a season, but liquid algaecide and clarifier both weaken once the seal is broken and a jug that has spent a summer in a hot shed is not worth dosing a whole pool with. Dry shock keeps better than anything else in the box. If it has separated, gone cloudy, or changed colour, it goes out rather than in.