How often do i do water changes?

OK i have two tanks. A 35 gallon and a 10 gallon. In the 35 gallon i have a Top-Fin 60 filter,and a million-air pump for an airstone. I have 3 koi and 2 goldfish in there.Yes im going to give the koi away because not enough room. And in the 10 gallon i have a male betta with a Aqua-Tech 5-15 power filter and a million-air pump for an airstone. How much percent of water changes for the 35 and 10?o and i also have a heater for the betta.