I just done my 2----8 gal tanks. Do not clean the filter.......that has all the good bacteria in it. Just unplug it and put to the side. Put the fish in a bucket or container and keep them warm (even take the heater and run it in the bucket). Save as much of have the water out of the tank if you can. Change ur substrate and fill half with tap water at the same temperature you are keeping ur fish at. Then add old water and fish. Do this will save the most of bacteria and you should only mini cycle. I do this all the time and never lose fish. Make sure you do not change filter, this is carrying most of ur good guy bacteria.