I have 2 goldfish and one is 5 inches long and the other is 6 inches. They are 7 or 8 years old and live in a 20 gallon tank. The goldfish when my grandfather got them were just the 20 cent feeder goldfish at a petstore. Now there bigger like I said and am wondering If I can move them to a 10 gallon tank If I got a really nice filter. Thanks.