Flocculant
TFP discourages the use of flocculants with ALL filter medium.
Flocculant is a glue that causes particles in the pool water to clump together and fall to the bottom of the pool. Flocculent does not get rid of live algae, or lower CH, or remove CYA from the water.
Flocculants are meant to be removed by vacuuming to waste. You add them to the pool and then only recirculate long enough to distribute the chemical (usually an hour or two). Then you shut off the circulation and let the floc fall to the bottom of the pool (typically 24 hours later). You then vacuum the floc to waste to remove it.
Flocculants should only be used if you can vacuum to waste. Vacuuming flocculant into a filter can gum up the filter, require major cleaning of a sand filter or DE filter, and destroy a cartridge.
If you leave a filter in filter-mode, some of the floc'd particulates will make their way into the filter and get trapped. Floc's do not last forever, they begin to break down back into their smaller particle sizes within a few days (there's an entire technical discussion of why and how this happens that is not necessary to get into here). So, any fine particles trapped in the filter as floc will eventually start to break up again and then come out of the filter. It won't be terribly obvious but what you will see is cloudy water developing slowly over time.
The circumstances where floc actually fixes something that won't fix itself with a little patience are extremely rare. If it works perfectly it shortens the clearing time slightly. If it fails, you can be dealing with huge consequences in both time and money. Not to mention it can mask ongoing issues that otherwise would have been properly fixed.[1]
Flocculant is almost never entirely removed from the water after vacuuming to waste and causes cartridges to become almost useless. Sand filters can more readily recover from a proper floc treatment, but long-term it will ruin those too.
One of the few times TFP recommended the use of a flocculent was to clear a pool of ashes from a wildfire. We advise to exhaust every other possibility before using a flocculant. Floc's and clarifiers can be tricky to use and, if used incorrectly, they can cause a royal mess. So they are definitely a method of last resort.
Flocculent is different then a clarifier. Clarifiers will cause small particles to clump together so they can be skimmed out and captured by the filter. Flocculents cause larger particles to glue together and fall to the pool floor. Flocculents need to be vacuumed to waste and never to the filter.
If you have already added flocculent to your pool and let it into your filter:
- for a sand filter, there is a great chance you will need to change your sand
- Cartridges need to be replaced in a cartridge filter
- DE filters need to opened and the grids cleaned.
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