Does it have a filter? If not I would do 50% water changes every 2 day's. When you do the change;s, If you can siphon from the bottom so it's removing the junk at the same time your changing water.. If it has a filter i would do 25% changes every week. And still siphon from the bottom when doing so... Again this is what i would do. Each person is different, Each tank is different..