They still exist, I've even seen them for sale once in while. You can pick up diatom powder (but it usually has to be special ordered). I dont think many people use them anymore. Basically the idea was to strip the water of anything and everything in hopes of keeping it crystal clear. We also used to take all the LR
or dead corals out and bleach them white again every couple of months. Now days everyone runs a much healthier, more natural system, so the use of the diatom filters has fallen to the way side. I've still pulled mine out and use it on occassion to deal with issues like dinoflagellates or when I had calupera go sexual on me in a tank, but otherwise it sits unused in my cabinet.