If you can afford to use RO
, that is the ideal. No concerns about what might be in the water or long term problems. If you can not use either, tap water is probably not as terrible as some might claim. I have used tap, and know several others that have, in both reef and FOWLRs in the past with no noticeable problems. I just filled my 240g with treated tap water. I plan to top off with RO
(the tank came with an RO
unit)...I just need to hook it up at some point.