Each and every last ich parasite would have to pass through the diatom filter to completely eliminate it from the tank. The chances of this happening will depend on the flow rate, the size of tank and the number of crevices and low flow spots where the free-swimming parasites can avoid being sucked into the diatom filter.
As you can imagine, in a running display tank with live rock and substrate it would be impossible to for the filter to remove ALL parasites at any one time. It only takes a single parasite missed by the filter during the free-swimming stage to restart the whole life-cycle all over again.
The following quote from Steven Pro's excellent article on marine ich illustrates my point:
"There is another interesting observation I found in my investigations concerning the biology of Cryptocaryon irritans. Mature trophonts leave the host and tomites exit the theront/cyst in the dark (Yoshinaga & Dickerson, 1994). Imagine if you will, a fish that randomly acquires a single Ich parasite. After a couple of days when the trophont is well fed, it prepares to drop off its host but waits for the environmental trigger of darkness. Meanwhile, the fish prepares to "bed down" in its favorite hiding spot in the aquarium; the same fish occupy the same spot practically every night. Now, the trophont leaves the fish, encysts, and begins to multiply. Several days to weeks go by and that same fish returns to its same spot at night, only this time there are hundreds of infectious theronts seeking out a host/victim in the same area. I am sure some of you are thinking that this is absolutely diabolical. Others can appreciate the simple beauty of this plan. To me, it is just another reminder of how remarkable evolution and adaptation is."
In other words, the diatom filter sitting at the opposite corner of the tank can't get to the beasties waiting to infect your fish underneath a crevice where's there's little flow.
Put yet another way: if you installed a magic air filter in your house that could filter out all bacteria and viruses, but someone walks in with a cold and sneezes in your face, you'd still get sick.
The diatom filter method would work best in a QT-style setting without rock or substrate where there would be no places for the parasite to hide. This, in conjunction, with hyposalinity and water changes would ensure the ALL parasites are eliminated.
Diatom filters would also be harmful if used longterm in a reef tank because it would filter out the many of the micro-organisms that corals (and other creatures) feed upon.
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