You mentioned that this behavior has started over the last few days? Other then sitting on the bottom of the tank, any other signs? Fast breathing, signs of infection, sores, etc. on the fish? Has it stopped eating?
Agree, test ammonia, nitrite, nitrate. Test all three, if you had an ammonia spike in last three days, the other two will show higher levels.
Also look at salinity, temp, etc.
Make sure all equipment is working as it should be.
Any changes to tank in last two weeks?
New additions, etc. ?
Anything out of the ordinary done to the tank?
The more info you provide the easier it will be to help you out with suggestions.
