If you are doing a fishless in the 25 gal, go ahead and put the proper sized filter on it. It may slow down the process because you've removed the bacteria that was present on the mini but there should be some on the substrate that will help the new filter to colonize. The mini that you swich over to the 20 gal should give that tank a little jump start in its fishless cycle as well as long as you don't let it dry out or add it to untreated water. If the 20 isn't ready yet just run both of your filters in the 25 till it is.
ALASKA
ok i started to cycle a 25 gallon tank on april 2nd with a aqua clear mini was told that was good enough for the tank and have since learned that it is one size to small for the 25 i won a 2nd 20 gallon at the lfs at easter if i take the mini put it in the 20 and put the 150 put it in the 25 will that be ok was told with the bacteria in the water that it would be ok and the bacteria in the mini may speed up the 20 or should i wait till this tank is fully cycled and established 
