Don't know your filtration setup, what or how much you feed, or sizes or numbers on all those fish, all of which would help to understand your tank. But that's an odd assortment and a lot of waste from some of those species, such as the Oscar and sailfin pleco.
Yes, increasing water changes is a start, keeping filter(s) clean also important. But a really good answer practically begs deconstructing your setup (in other words, more information) to analyze it better. Your GT looks underfed-- something I rarely say. I'm not saying to feed more, don't know what your other fish look like. It could be the tank overall is getting plenty but the GT is stressed, has a health issue, is being outcompeted for food, being bothered by one or more of the other fish, etc. In such cases simply feeding more wouldn't be the answer.
At some point, imo, what's needed is to assess what kind of tank you really want, what place the GT will have in it, and reorganizing or refocusing the tank to better fit its capacity. GTs (or their close relatives) normally grow faster than that in favorable circumstances.