Okay, try to up the water changes to at least 25% if you can. The more you take out, the more ammonia you take out. The idea is to test daily while it's cycling and when you see at or more than .25ppm then you do a water change, wait an hour and test again. If you still see ammonia, you have to do another change and another if it calls for it until the ammonia is down or gone. You're going to really stress out the fish if you don't and because the tank isn't cycled and you have a lot of fish in there, it's not going to help. I suggest you get another filter rated for 20 gallons at least, the 10-15 is not sufficient for the 20 unfortunately and it's not going to be able to let enough BB colonize there to let the cycle run properly.
Beneficial Bacteria (BB) will colonize on any surface but the more surface area you have, the more BB you can have to keep your fish safe.