Floc is short for flocculant. Flocculants are a chemical that make small particles join with each other to make bigger particles. This is handy for swimming pool owners because their pools tend to get little bits of dead algae and skin in them that make the water go cloudy and these can be too small for the filter to strain out.

There are two main types of flocculant but they are both the same chemical - Aluminium Sulphate also know as just plain 'alum'. In one form it is in flakey granules in the other it is a solid tablet (kibbled alum). The latter is used to 'polish' up a pool that is getting a bit cloudy the former as a last resort on a truly mucky pool (or as a first resort when opening up after winter).

Kibbled alum tablets are added to the water via the skimmer basket. Don't put them in the circulation pump strainer basket because the water in there goes to fast for them. Keep your filter going full time. The alum dissolves and forms a coating on top of the filter sand. Then, as the little particles come by it grabs them and keeps them. But in doing so it blocks up the top of the filter and can increase the filter pressure. If the pressure gets too high the coating breaks up and gets washed through the filter and back into the pool taking all the little bits with it and you are back to square one. So, always thoroughly backwash the filter before putting the alum in and keep an eye on the pressure while it is in there. If it gets too high backwash add more tablets. In any case backwash after 48hrs of filtration and if the pool isn't clear enough do it again. It may take three or four goes before you get it perfectly clear.

There are other chemicals you can use to clarify your pool, the term floc tends to apply to Alum. Pool Clarifiers, as they are usually called, do the same as Alum in that they gather small particles together but they work by putting an elecrical charge on the particles. The general name for them is Cationic Liquid Clarifiers and there are many types. They work very well and quite quickly but are much more expensive than Alum. They now make them in a gel form that makes them easier to handle and use but no less expensive.

A newcomer to the clarifier market is Sea Klear. This is a natural product made from crustacean shells. More expensive than alum but much cheaper than other liquid clarifiers it is a very good clarifier and is proving very popular.