The more available space the better. I had two Ebjd in a 20 high for a month didn't grow any then I put em in my 40b they grew like weeds.A 20 gallon is plenty big enough for a 1" fish. The only problem is that he should have been in a tank by himself or with his own kind to grow out. I've heard that other species of fish release different hormones that could potentially stunt the smaller fishes growth, especially in that small if a tank. If you have a bigger tank that you can move the other bigger fish to and leave him by himself, I would try that for a month and see if he grows a little. If not, he's destined to be a runt.
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