This past week I lost the goldfish that this post was originally about. The fish got what I can only describe as white, stringy (almost silky?) fungus/bacteria on its eyes/gills/tips of tail. It looked like the fish swam through a cobweb, honestly. Found it dead early last week. I got it out of the tank immediately. Did an 80% water change, looked closely at my other two fish (another goldfish and plecostomus), they seemed to be unaffected. I didn't take a water reading because I figured the results were irrelevant since a dead fish was contaminating my water.
A couple days later ( believe it was Wednesday), the white goldfish developed a swim bladder issue and then overnight had developed the same type of stuff on it. It also at that point had developed tail rot, and I could see the veins in his tail. They were red under the skin. I really at that point thought I had lost another fish. I vacuumed the gravel out again and did roughly a 60% water change. The plecostomus during this time was fine. It was going about his normal glass sucking/cleaning routine. Water readings were all ok, except my ammonia was a little high at ~ .25 ppm.
Thursday my white goldfish was able to swim upright again, and did not have any more signs of the white stuff on it. Again, plecostomus looked normal.
However, on Friday my plecostomus was over in the corner of my tank laying with its side on the gravel, and leaning its back against one of the rocks. It was not sucking on the glass. At first I thought it was dead, but I watched and it was breathing. There are not any visible signs of the same thing that affected the goldfish, but I'm sure there is a possibility that it would show the same signs in a different kind of fish.
Today, my white goldfish is swimming around normally. It's eating and I cannot see the veins in its tail anymore. It also looks like its tail might be healing (it is looking better than it did the other days). The plecostomus is still laying in roughly the same area, still breathing, but I doubt it has eaten much at all over the weekend. I did another water change, this time of 50%. I still had ammonia levels of ~ .25 ppm.
Could the plecostomus have eaten something bad that was kicked up from when I cleaned out the gravel? I'm not sure what could be wrong with him.