Ro water contains less contaminants..Doesn't matter if it's a reef or fish only... evena fish only can develope slime bacteria etc..with bad water..most city water is high in phosphates which can cause bacterial problems in the long run and get expensive to get eliminate...GO with ro or you can buy and ro,,,ro/di unit For under 200.00 well worth it,,It will save you money in the long run ..even over buying water..