will a 10- 15 gallon filter filter a 20 gallon tank?
It will, but will not do a very good job. You want to filter 10 times your total gallons per hour. So a good filter would filter 100 gals. per hour on a 10 gallon tank. If you want to boost your filtration cheaply, I would suggest a sponge filter that is rated for your size tank. They are probably no more than $20.00, and I have them in almost all of my tanks. They are economical, provide great biological filtration, are easy to clean, are safe when fry are in the tank, don't lose their prime when you have a power outage, and if you get the kind where the filters can be removed from their holder, you can actually take one from an established tank, and use it to see another tank. Of course, you will need airline tubing and a good air pump, but they are still great filters.
What you will want to do and its what I do is to. Well I use whispher filters and heaters. And for my filters on the box it tells you how many gallons it can filter so for example I have a long 20 gallon tank and I got a filter that filters 20 to 40 gallon aquariums. So the thing to do is to get a filter that starts at what you tank size is and goes up to a higher size tank like I do.
Good point Durbkat! Also, try not to go by what the manufacturer says on the box, but go by the gallons per hour (gph) rating. You want to try and get 200 gph for your tank. I know it sounds like a lot, but you will see what I mean once you do this. Your fishies will thank you for it!
i hav a penguin biowheel for a 50 gallon tank in my 30 gallon tank and it filters i think a couple hundered gallons an hour i htkn in the 200's not sure but i lkove it and so do my fish definatly over filter ur tank but not to much that the current is to much for ur fish