We set up a new 75-gal tank two weeks ago for our 3-yr-old, 9-inch comet goldfish, 3-yr-old, 11-inch pleco, and 20 glofish Danios. Tank has live plants and two filters, one rated for a 75-gal tank and the other for a 125-gal tank.
Water test shows: 0 Nitrate, 0 Nitrite, 75 GH, 0 Chlorine, 80 KH, and 7.6 pH. Water temp is 76 degrees.
Two days ago, my goldfish and my pleco stopped eating and became lethargic. A few times, I saw the goldfish flashing himself against the gravel in the tank. Yesterday he developed red, irritated-looking veins on his fins and tail. As you can see in the photos, his fins are now very red. I suspected ammonia stress and did a 25% water change last night, and another 25% water change this morning. The Danios have been completely fine the entire time, showing no symptoms of stress.
Should I relocate the goldfish (and the pleco?) to a different tank, or just continue with water changes?