Don't worry every new saltwater hobbyist has made this mistake even I have. I just want to tell people about it so you don't learn the hard way. As you know or find out the evaporation rate in saltwater tanks is alot. My 7 gallon nano needs water every other day.
But here is the mistake many make and can actually kill your fish and it has nothing to do with chlorine, nitrates, metals or anything like that. It's about the salinity of the water you are adding. You do not add salt to the water you are refilling for evaporation. Only add salt when you take water out of the tank. When water evaporates out of your tank it leaves the salt in the water. So only add freshwater that is filtered, RO, or purified.
If you add saltwater when filling for evaporation then you will raise the salinity to the point where it kills your corals, fish, and other living things in your tank. Just keep all this in mind.
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