a 300 gallon is PLENTY big for a pair of Dovii, a 500 to 600 gallon is quite excessive. I know a very reputable breeder of Umbee and Dovii and keeps his pairs in a 300 gallon. Mike Mann is his name, you can find plenty of his videos on youtube. His might not even be in a 300, they might even be 220s and his fish are extremely healthy, grow like weeds and produce constant spawns. He is sadly getting out of the hobby for some reason though. I'm hoping to buy one of his 300 gallons here soon, just have to keep my fingers crossed that someone backs out of buying one of them.