Sounds like there are deeper issues . Since no one has said it it may well be the tang .. He is going to get too big for a 9 gallon and they carry ich like a magnet .. and while you may not see the spots it wont be long before they show if they become stressed .. 3 fish in a 9 gallon with one being a tang is way over maxed out ..
It sounds like you are not ready for fish as well .
You should be testing for :
NitrAtes : These should be less than 20 with corals
Ph , will change a bit with lights on and off .. It will be higher with lights on than off ..
and your trace minerals since you have corals .
SPG 1.024-1.026 with 1.027 being on the high end of acceptable ..
You should also have a temp probe in your tank to coincide with the heater ..I run my tank with fans and an open top (with egg crate) at 77*-78* with a one degree variance, + - controlled with the RK2 ...
I run 2 diffrent thermometers in my tank .. 1 is on the RK2 and the other is a digital thermometer .. I also have a meat thermometer (digital) to do a calibration with if I suspect something is up .
If your top is closed you may be having oxygen issues as well .
Doing the water changes is good but there are other things needed ..
Clowns are funny ... My 2 that I have paired so to speak but not fully , they will go for a week as buddies then seperate . Just watch because one may be come over dominate and kill the other after all they are damsels HTH