You SHOULD use ro
water, or distilled, but, like I said, if you premix the water, oxygenate and age the water you can use tap, as long as it's stable and balanced in spec. Make sure you check once oxygenated and test once again before adding the water, you can keep a smaller tank in check. I have a puffer FOWLR
and am starting a nano reef... It's a ton of work, because, yes, I use 5g buckets, lol. I don't have any tanks above 40g.