Just straight up tap water?? Really? I think someone was mistaken. Tap water contains way to many different things for it to be used without being treated and filtered. Various contaminates, metals and chemicals like chlorine and also phosphates. While you could get away with using tap for a freshwater tank (still has to be treated in some ways) I haven't heard anyone say it's ok for a reef tank which is why people buy water from the LFS
get distilled water or invest in RO
Edit: I did some quick reading and to my surprise some people claim to use only tap water for their reef tanks. If you are considering starting up a reef tank many people will tell you to not chance it and just get an RO
unit and save yourself the trouble..myself included. Just using tap water can be a huge hazard to your tank and personally not worth the risk in my eyes. You can request a report from your local water company for the water you receive and you'll be able to see what's in it. The accuracy of the report may still be skewed depending on the pipes in your home and their age in regards to metals like copper. Regardless the water is going to contain some things that you'll need to get out of the water before you use it like chlorine or chloramine.
If you are going to spend all the time and money on the tank I wouldn't ever chance it by using water that wasn't as "safe" as possible.
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