I use a "Mighty Mite" from a place called Air water ice on line. It costs about $100. It hooks up to any sink easily. I use it to change water on my discus tank cause they are so sensitive. I make the water in a big clean Rubbermaid trash can. Put heater and bubbler in the can to have it ready for the fish and then use a pump to pump it to the tank. This is a little RO unit that makes a trash can full in about eight hours. It works great for my purposes.