I’m not because I use the store bought water and do weekly 10% partial water changes. I haven’t had to add any water beyond that. Is that wrong?My money would be on not enough oxygenation. I would definitely start with increasing the surface agitation.
Also, I noticed you said you didn’t check salinity from the lfs, but you do check your tanks salinity periodically dont you? This may seem like a stupid question but when you have water evaporation you are replacing it with fresh water not saltwater aren’t you?