Ed Schwing wrote:

We have a 43 gallon freshwater aquarium, with all the necessary filters,
pumps, etc. Our tank has been set up for over 12 months, and we have had 4
fish in the tank for just about the entire period. The water quality has
consistently checked out, I do partial water changes (30 to 40 percent) at
least every 5 to 6 weeks. Our water is, and has always been, pristine in
appearance too. We have one nice black moor, and three orandas, 1 redtop, 1
bluetop, and the other was supposed to be a blue also; however, he changed
to a golden color.

Just as a pointer - 30-40% every 4 or 5 weeks is probably to infrequent.
You should most likely be doing 30% every week. Get a nitrate test kit to
ensure you're levels are low