Arvind, I have a pretty big tank with pretty big fish so going fallow for me is a non-starter. I have encountered both ich and velvet many times and unless you're caught flat-footed there is no reason to lose fish to these infestations. These parasites are big. They are easily filtered out and no you don't have to get them all. You just need to reduce their numbers to below the lethal threshold until your fishes natural immunity kicks in...this coupled with cellular senescence will pretty much accomplish what your trying to achieve by going fallow.
The filter doesn't need to be diatomaceous earth. A simple string or foam water filter would work on a smaller system. What is most important is that you use the filter appropriately, with adequate flow and to do this you need to understand a little about the parasite and how and where it acquires your fish. Read: Dynamics and distribution properties of theronts of the parasitic ciliate Cryptocaryon irritans. From this article you should get a better sense of what you're dealing with and will be able determine those areas in your where your fish will most likely become infested. Address those with good mechanical filtration and barriers if necessary and most of the work is done.
Now on your next system build, include the capacity for solid mechanical filtration in your build and you'll lose a lot fewer fish.
The filter doesn't need to be diatomaceous earth. A simple string or foam water filter would work on a smaller system. What is most important is that you use the filter appropriately, with adequate flow and to do this you need to understand a little about the parasite and how and where it acquires your fish. Read: Dynamics and distribution properties of theronts of the parasitic ciliate Cryptocaryon irritans. From this article you should get a better sense of what you're dealing with and will be able determine those areas in your where your fish will most likely become infested. Address those with good mechanical filtration and barriers if necessary and most of the work is done.
Now on your next system build, include the capacity for solid mechanical filtration in your build and you'll lose a lot fewer fish.