that's terrible. top-offs usually have to be done every day, particularly if you have inverts. they are VERY sensitive to salinity changes, especially creatues like anemones. In a FO
tank, it's not as dramatic, but still stresses the fish.
I top off daily, and in doing-so I dose my alk
buffer and calcium supplement (I do have a reef, not a FO
I have a fuge, so I usually pour FW
in there, and then some in the display tank, near a powerhead so it mixes more quickly. Corals, more than fish, will suffer from being in FW
, but in tanks with proper circulation, it should mix in so quickly that the effect on salinity is negligible. When in doubt, just pour more slowly, or do a.m. and p.m. top-offs.