My little girl has been given a small tank and two goldfish. The tank is "established" ?? ie the fish are about a year old and arrived in a bag with there own water. I've put them in the tank with some fresh treated water. There's a pump. instructions from previous owner is feed them twice a day and change water regularly at 1/3 every week. What else do I need to do to ensure we don't shorten there lives too much.