Sounds to me like a few problems you might have are:
* Oxygen saturation
Fish need to breathe air. If you're not getting good gas exchange (carbon dioxide releases from the surface of the water and allows oxygen in) your fish may suffocate.
Corals need it (don't know about fish). Flow will improve the health of everything in your tank. It will also help agitate the surface water to allow for gas exchange
* Top offs
Mark a line somehow where the water level is. Marker, tape, whatever. Every day add RO
water (or dechlorinated tap water if you must for some reason - ammonia in tap water will kill fish and corals) back up to the line to replace water that has evaporated. In a ten gallon tank this can mean a huge difference in salinity. Big fluctuations in salinity will kill fish and corals. Salt doesn't evaporate, only water does. So when evaporation occurs, the water has higher salnity. Replacing the evaporated water with pure water brings the salinity back to normal. Doing it daily prevents huge fluctuations.
I would get a powerhead to move water around and agitate the surface of the water. I'm not saying it will solve the particular problem you are having (it could always be something else) but it will improve the overall health of your system.
I have two koralia 240gph powerheads in my 10 gallon and a 100gph return line from the sump. This is a lot of flow, you could get away with way less.