Hi, they can get up to 12 inches I think. Let me explain this a little nicer. The golds fish will nibble on the axie gills cause them stress and most likely they will get fungus. The axies will also try and eat the gold fish. They will try and eat any thing. When they eat they might, will at some point eat some of those pebbles that are in it's tank. They will get stuck in the axies cause all kind of problems. You could use play sand or pool filter sand for the bottom. I just changed mine to a bare bottom tank it is vary easy to clean.