That looks like a good Filter. Just keep in mind it's a use & remove item. Not something you leave hooked up all the time.
How long have the 3 clowns been in the tank?
I agree with everything above:
Low flow.. need more movement in the tank. I have 1x 1800gph pump,
2x1200gph pumps in my 90g tank and want more.
Over Feeding That's a lot of food for what you've noted as in the tank. That's more than I feed a fully stocked tank with many more times fish than you have.
Just take it slow and make mild adjustments to your system and don't try adding any more chemicals to it. It will work itself out over time.
Hang in there
