That's up to you. Since your fish are out and I assume you have a suitable home for them you could complete the fallow period. I personally wouldn't go over a month for ich or velvet, I would stir or sterilize my sand-bed, but whatever you do, I would suggest you to look closely at your system. Your tank is probably set-up in such a way that it naturally becomes a death-trap once ich or velvet is introduced. That's not a knock against you, its just that most tanks are.
I like to use sinking fish pellets (food) like Hikari "A" and Hikari "S" as parasite proxies. Drop some in and watch. In most tanks they may drift around a bit, but once they hit the sand-bed, they pretty much stay in place or worse yet accumulate in all those places where your fish settle down to sleep at night. That's typical of tanks where the only draw is the overflow and flow is positioned to "not disturb" the sand-bed. Now you can see that if instead of food, if those pellets were ich or velvet, your tank would very soon develop a major problem. Fix it. You want the pellets to hit the bottom and keep moving ultimately ending up in an area where you have positioned "deep draw" straight to a filter or have created "turbulent lift" up to the overflow. Either works - deep draw is old-school and easier but turbulent lift works too.
This is actually easier than it sounds. In a rectangular tank, I like to position return flow length-wise in the tank - pushing water away from the overflow. When it hits the opposite end the current will drop and return along the bottom to the base of the overflow. Make sure you don't obstruct these return paths along the bottom with rock etc. I position my rock in the center, down the length of the tank, ensuring free flow along the bottom in front and behind my rock work. Where pellets are caught by the rock - position power-heads to blow them back into the main currents - front and back along the bottom towards your deep return. Bare-bottom is easiest obviously. Fine sand moves around in the flow. Course sand or gravel is fine especially in conjunction with under gravel filtration. UGF or RUGF both can be used to advantage.
I don't care what type of filter you run - DE or otherwise - this flow has to be right or all your doing is stirring some very clear water.
Pool DE filters are fine. Cheap and powerful. One can be had for ~$200 which will work for tanks up to 500G. Most salt tolerant pool pumps can be used as well - multi or variable speed - they won't leak diatom powder back to the tank unless they're installed wrong or broken. You don't need a pre-filter nor would I think you would want one. (I don't have a pre-filter on my tank or my pool for that matter). They will strip plankton and substantial bacteria from the water - unless you know exactly what bacteria you have and why - this is a good thing - (lots of potential nasties - read the link) you can shut the DE down to feed corals etc and clear the excess in minutes. You can also easily run carbon along with DE powder. Comes in handy.
I run 24x7 - mostly at low speed - but if something happens, I can open it up and clear > 99% of the water in minutes. I have another post that helps you figure the flow rates.
Ich and velvet are not the same but the have a similar 4 stage life-cycle with differences in stage duration etc. Velvet is fast - completing a life-cycle in days - ich takes twice as long. There are numerous strains of each - genetically quite different and distinct, but it turns out that a fish that develops immunity against one strain generally is protected from other strains of as well. The fishes immune response builds quickly and persists for six months before it wanes. Different species of fish have different innate levels of susceptibility - some bottom dwellers have a resistance (different than immunity) usually attributed to their slime coat or scale structure. Other organisms - shrimp, snails, pods suffer from similar parasites. It is believed species differentiation is a function of the parasites specific attachment mechanism for their hosts. It takes a fairly large number of parasites to kill a fish, on the order of hundreds per gram.
Healthy fish have healthy immune systems, but they don't have immunity unless exposed. You can't feed your fish to be immune to ich or velvet unless they come in contact with the pathogen. Nice idea though.
Vortex DE filters are pretty crappy, low quality and unwieldy - The magnum HOT is a toy, I wouldn't bother with either except on the smallest of tanks or out of utter desperation.
Most who comment on DE filters only know the vortex - so of course they suck. Most others have no idea how to implement them, but they're used in commercial operations and public aquariums routinely. That's where I learned of them, from a marine biologist who did much of the early research on velvet and ich, who consulted and did research for commercial fisheries and who was also the director of a public aquarium. Guys name was Dr. Adrian Lawler
https://www.reefs.org/library/talklog/a_lawler_111697.html
Proper use does take some work and practice. It took me a while to get the configuration right. Now its easier to service and operate than a skimmer - less than 10 minutes to flush, charge and back online. Never overflows - no adjustments - no need for fallow periods - I don't quarantine (although admittedly I should). My son got me a huge Black Tang which came in absolutely covered with ich. Really bad shape, went straight in - recovered - no additional casualties. Got a stubborn strain of velvet introduced with an Achilles Tang also in bad shape. Three days on high flow took it out as well. Corals seem to do well - at least no worse.
With all that said - it doesn't have to be DE - DE is just the easiest cheapest and arguably the best mechanical filtration I've ever found as it addresses one of the major issues in reef-keeping - instant dilution.
