I've got a few dempseys in a small outdoor pond. Very cold resistant fish, have had them down to 65 deg without any problems. Normally they are kept at 72+ deg.
They need some room if you keep a group of them. Once they start to pair off they will need space from each other. Visual barriers help, as do dither fish like tinfoils or similar to displace the aggression a bit.
Messy eaters, need good filtration and lots of big water changes. Our largest males are about 7 to 8" and they lose as much food as they swallow, like an oscar would.
You will end up with way too many fish eventually even from one pair. They don't sell well, supply and demand, so plan on giving them to friends or use as feeders. Ours disappear from time to time due to egrets and herons, or a suicide trip over the side of the pond. Or you could get some good size catfish to keep the poulation under control nocturnally?
Dempseys are a great fish, easy to care for and fantastic colors on males and females. Enjoy.
