Taken from the Vortex diatom filter website....
"The Diatom Filter will filter out particles and parasitic life forms down to about one micron in size. We are not particularly after the bacteria. We are after the fish killing parasitic protozoan life forms such Ishthyophtririus Multifiliis (Ich), Chilondonella, Myzobulus Sporozoan, Octonitiasis and other Flagellates, Sporozoa, Leaches, Etc.
It is almost impossible to get rid of all the parasites in an aquarium, but with the proper filtration we can keep the population of parasites low enough so they do not harm healthy fish. Medication sometimes helps, but I am a firm believer that more fish are killed by over treatment with medication than any other cause. Proper filtration is the key.
Oxygen molecules are on the order of 4 Angstroms is size and are unfilterable in the general sense of the word. The Diatom will not filter out the oxygen, in fact it actually reduces carbon dioxide levels in the aquarium because of the speed of the impeller (3,000 R.P.M.). Dissolved gases are liberated and allowed to escape at an accelerated rate."
Some people love them, some people don't believe in them, I wouldn't go without one.
Here is the one I use...
http://www.diatomfilter.com/products/diatom_xl_filter.htm
