is tap water really that bad?

[GaryP]

Yes calcium and mag are replaced by sodium. So you you topoff, water evaporates sodiom does not. Repeat repeat repeat repeat....sg goes up.

OK, you are absolutely correct. I'm sorry, I misunderstood your comment. I thought you were saying that softened water was worse then tap water. Pretty much the same without the benefit of the calcium and magnesium.

[Richard]

That is the same type of thing I'm talking about when I tell people they can drive their alkalinity too high by always topping off with tap water.

I had a guy who was losing his african cichlids from having auto topoff on his sump hooked up to tap water. After going through 3 cc's of the salifert reagent without a color change I gave up trying to measure his alk and just told him he need an ro unit for his auto topoff. That is the first and only time I've ever seen or even heard of anyone having alk too high for african cichlids.

Of course the carbonates are used up faster in a reef than a fw tank but if you lose 4-5 gallons per day like my tank and and your carbonate demand isn't real high then the same type of thing could happen.