As stated it does depend, Id say quite a while though being ocells, I had an ocell for half a year and he went from 3 to 7 inches, I dont know how big he would have gotten at the end of the year though, I would say they would be happy until they get to about 15-16inches being that the tank is only 18inches wide, which I believe females don't get that large so if you have two females you're set, thats my personal opinion. I would say anything at 180gals or larger would work for life with ocells, but once again thats my opinion.