i agree with reef, i was using tap water when i first set up, now i have a RO-DI, would not be without it, rates in my top 5 best peace of equipment for my tank, the tap water in hamilton is full of bad stuff, get it tested at a lab and they will tell you the same thing, defiantly look into getting a water filter for top off, or use bottled water, packnsave have bottled water in 5ltr packs that is RO-DI, costs about $4 from memory.