I used these all the time in my graduate work to filter leftover algae we feed to our study animals. We used 5 gallon buckets and it took about 15-20 minutes to clear 5 gallons. On my 180 gallon display to mimic the dilution how much turnover through this filter would I need? Just don't think it would be very practical or cost effective on larger displays unless you have a large dedicated room to house the filtration. I don't doubt there ability to remove these just don't know if it would remove them fast enough without some seriously large turnover of tank volume in addition to our normal filtration turnover."Hard mechanical filteration" is the easiest (and maybe only) way to mimic the dilution effect of the ocean in our systems. Its often over looked and does take some planning to implement. The nice part is that it is effective against all pathogens which posess a free swimming stage. It doesn't mater if its ich, velvet, a toxic algae, fluke or worm - or even if you have no idea what it is - or how it came to be in the system. It places no negative load on your inhabitants and carries no toxicity. It limits bacterial populations and reduces secondary infection resulting from any source, whether it be disease, stings, collection damage or in tank agression.
A Diatom filter is not the only effective form of filtration, but it is prehaps among the best. For ich and velvet which are physically quite large, a suitably sized paper filter would be sufficient provided the required flow and tank configuration are acheived.
If you have ich or velvet to an extent that fish die, it is an indication that the tank lacks sufficient filteration or is pooly setup for its intended inhabitants. Resolve these issues and you go a long way in preventing future problems which you have not yet encountered. Quarantine, TTM and fallow practices may forstall an infection - but they are reactionary and don't address the fundamental cause of the problem which is the lack of "dilution" present in too many of our systems.