Sounds like water changes to me. This will combat what I'd see as 2 possible issues.
One of this would be the build up of nutrients, which is pretty straight forward.
Second would be such like alkalinity. In my 55, my sps
did well and my polyps began to decline. After some testing it turned out that my alk
was out of whack. I did water changes and did some dosing of alk
to get it back where it needed to be and things returned t normal. You don't need to dose, as water changes will solve the issue. This was just a step I took as I've struggled with alk
before in the history of this setup.