Enough information has not been given on how the fish are acting, the breathing, gill plates color and extention or not, etc. so based on the limited information i will provide you with some possibles to zero in on i.e.
1-Chilonodella
2-columnaris
3-amyloodinum
4-bacterial infection
5-Poision i.e. are there poisonous fish or animals in the tank (rabbit fish) do you have any reason to believe they may have released their poison or tung the affected fish? new landscape objects or equipkent recently added, test for ammonia, nitrite, and nitreate if high could be problemany perfumes, insecticides, cleaning chemicals, detergents, excessibe bobacco smoke, oils or strong vapors been released into the tank water or sprayed near the tank or air intake recently. good luck hope something here helps.
6- gill and or skin flukes