My way of changing water : 1. I fill a 160 l (40 gal) tank (water butt) with reversed osmosis water, and add about 20 l (5 gal) of tap water. 2. I heat this water up to 20 °C. 3. I switch of pumps and heating of the tank, and let flow the same amount of water out of my tank (directly in the drain) ...