Ok so my tank is 4 months old. I'm having normal algae issues but getting better. CUC is lacking at the moment, but will add more soon. I have had trouble adding fish twice now. Added 3 chromis, and one survived. They had been through 30 day QT, were eating well, and very active. After adding to the tank they seemed to have trouble breathing which always seems to happen when adding fish to a new tank so I thought nothing of it. Well two lasted a day, and the other 3 weeks.. Yesterday I woke up and noticed the other chromis having trouble breathing, and my clowns that have been in the tank over 3 months also having trouble breathing. First I panicked and started to get water ready for a water change. After starting water I moved the power head up to surface to get good oxygen exchange. While moving power head I noticed the skimmer was off, the container was full so it had shut off. I then emptied the container and got skimmer running. Within 30 minutes fish seem fine except the chromis. All shrimp and other CUC were not affected. Could this be from lack of oxygen in the water?? What can I do to help this?? Thanks