Does aquarium glass “go bad”…always cloudy?

I have a 20 gal tank that is home to an enourmous red-fin tin foil barb that is at least 5+ years old. I never expected it to grow this large or live this long. I put him into a clean but older aquarium about 2 years ago. Water changes are on a regular basis, chemicals are precise, filter is newer Whisper20. I find the water is cloudy a week or so after a water change. My husband thinks it’s something coming out of the glass. I’m tending to think it is the large fish producing a lot of waste because of its size. New tank?Bigger tank? Better filtration? Water clarifier?
Thanks for your input!