If the upsidedown cats are the only fish you have in there and it's only a 20 gallon, you can do everything in one shot, in less than an hour. Just put the cats in a fish bag with allot of tank water, then put the fish in a dark, so they can feel comfortable. Then save about 1/2 the water from the tank and keep the filter media in that same water. Then empty out the water and remove/replace the subtrate. Add the rest of the water, keeping the temp to about the same as the tank. When your done arranging everything, acclimate the fish to the water by floating the bag in the tank for awhile, adding some water form the tank into the bag every few minutes.
125 gal (473 liter): Low Tech
(1.5wpg PC for 10 hours, no CO2/ferts, gravel substrate), Equipment
(72" Coralife PC, Eheim Pro II 2128 w/built in heater, FilStar XP3 w/Hydor ETH201 inline heater), Fish
(6*Discus, 2*Angel, 5*Clown loaches, 4*L-018 Gold Nugget pleco, 1*L-260 Queen Arabesque pleco, 7*Cories, Farlowella cat)