I agree with tlindsey.
Any amount of waste added by more fish can inhibit growth (especially a waste maker machine like a goldfish).
Testing for nitrates can be your barometer.
If you add a new fish, and test for nitrate before a water change, and nitrates are higher than before you added the fish, its telling you to either remove the fish, or double up (again) your water changes, and water change volume.
In my tanks, my max nitrate level goal is no more than 5 ppm, so to achieve that level, I try to change 30% to 40% every other day (100% per week0
I collect fish in nature, and have yet to test a non-polluted source, where nitrate levels, are even mom-detectable.
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The above test are from different river in Panama, the left vials are all nitrate test, right are pH.