What a lovely tank!
A very standard water change program is 50% of the tank every week.
It could be many, many things ... Food, water parameters, stress from not having enough cover, stress from other fish ...
So here are several ideas:
It's critical you have a thermometer and a test kit. Check the temp at least daily, and the water at least weekly. Strips tend to be wrong, the API kit with the test tubes and bottles is good. It may be confusing at first but in time it becomes very easy.
You might consider some floating plants and a background on the tank to help the fish feel safer. It's a lovely tank and if a background doesn't work for you, lots more plants might. Or at least a few floating ones. I like brazilian pennywort and so does my betta (a gourami relative).
Food affects color. What food are you using?
I've also read about gouramis being given a lot of hormones to color up at the breeder, maybe that has worn off.
It's great that you noticed and came to ask questions!
Sent from my iPhone with three hands tied behind my back.