DE stands for Diatomaceous Earth. DE is the fossilized remains of diatoms, a type of hard-shelled algae. DE particles are microscopic and have many sharp edges, trapping fine dirt particles that would otherwise be able to get through the sand.
Small amounts of DE can be added to a sand filter after each backwash to improve the filtering. The first time you use DE, you need to figure out how much DE to add to your filter. After the first time, you can simply mix the full amount of DE with water in a bucket and pour that slowly into the skimmer.
Figuring out how much DE to use
Start by noting the PSI reading on the filter with the pump running.
Mix 1/4 cup of DE with water in a bucket and pour that slowly into the skimmer. Remove the skimmer sock, if any, before adding DE.
Give the system about two minutes to stabilize.
Note the pressure on the filter and repeat the second and third steps until the pressure has gone up by one PSI. This will typically take around 1-2 cups of DE.
The very first time you do this, you should keep an eye on the filter pressure for the next 24 hours or so. The improved filtering may quickly catch a lot of very fine particles and send the filter pressure up rapidly.
DE is a very fine powder which can cause skin irritation and is classified as carcinogenic when breathed in. Some areas prohibit the release of DE into sewers or the environment. Follow handling precautions as indicated on the product packaging. Gardening DE, while safer to handle, has not been heat treated and therefore is not suitable for use in pool filters.