SG question

[bv_reefer]

-I understand that when ur water evaporates u put freshwater and all because the water evaporates but not the salt but, but when u put freshwater doesn't it lower ur SG a bit? -cuz right now it's sittin' @ 1.022
and i was wondering if an liter of water cud drop it 2 about 1.021?

[surgeonfish]

You are correct. By adding fresh water (FW) you will lower your SG. However, with the initial evaporation of water (without loosing salt) your SG should have increased. Therefore, by adding FW you should be restoring (lowering) your SG to its previous value before the evaporation.

A SG of 1.021 would be ok for a fish only system. However, if you have inverts or coral that is too low. I would aim for 1.024-1.026. However, the most important thing is to increase or correct your SG slowly, particularly if you have inverts/corals. If you want to increase your SG now, add salt water instead of FW until your SG if where you want it, then keep up with your evaporation loss regularly to maintain a constant SG.
Good luck