Do I Have To?

[Mr.September15]

My tank has a major gap between the water line and the "Black lip". I'm going to buy some R/O water tommorow to fill this gap up. My question is, do I have to take out my fish while im doing it because its a lot of water and it might change the saltanity lvl lower. Or do I mix it before I put it in?

 

[Mr Miagi]

If you add RO water to a correct SG, the SG will lower.
If you add water at the correct SG, it shoudlnt change.

Only add FW RO to replace evaporation.

Mix salt with RO water before you put it in the display. Many people advocate leaving the new mixture a few days with a powerhead or some kind of water movement, because newly mixed SW is highly corrosive, and damaging to your inhabitants.

I hope I answered your Q.