I had to move all of my fish from my 36 gal tank into my 20 gallon tank. How often should I clean it?

do a 25% water change every other day and make sure the water is agitated also I'd recommend (if you don't have 1 already) to have air stones running (to make sure there's enough O2 in the water). I wouldnt keep them in the tank for long. Also dont be surprised if you have fish loss because of stress and extreme fluctuations in the water chemistry.