Hi, I have my return pump set at 2500lph for my 350 litre discus tank. When I do a large water change say 30% I turn the sump pump off, but I don't do this very often because I have a drip water changer on the tank which changes about 50 litres of water every 24 hours.
I also keep my discus tank at 28c all the time, it has dropped to 25c when I do a large water change but this doesn't seem to bother the discus too much and the water temp quickly rises back to 28c.