my way in changing water is that i keep a 25 L tank ready with filter and heater going on and i keep another one with a new water and leave it for 1 day .. next day i change 2 or 3 liters from every tank from the25L tank which is ready and going on ( with some guppy fish as for bacteria going on ) so when i am finish with changing water from the 25L tank i re-fill it with the 1 day old new water and so every day 