I had two in a 500 gallon tank. Way to small for them mine were bigger than basketballs...No joke. A 300 gallon anything is small.What about a 300 gallon pond? I think dimensions matter more than gallons. 5 feet of width is better than 12" or 18".
I would say a minimum of 300 gallon as a grow out and a 1000 gallon as a min to keep them forever.