I have a 56 Gallon freshwater tank with a filter capable of filtering 75 gallons per hour. I don't know the brand but I got it at Petsmart. I don't know the water perimeters but I have a PH reader and it is currently at 6.2 and really has been forever without any problems.
I have had this tank for 3+ years now. I've lost a few fish along the way and replaced them but currently I have two goldfish, one is 6 inches mouth to beginning of fin. Second gold fish is about 4 inches or so. I have two gouramis both at about 3 to 3.5 inches. All of this fish are over 2 years old. I have two Dalmatian Mollys about two inches each, two white Molly same length, 2 Mickey plattys about 1.5 inches each, two painted platties, four fry, one is a painted platy three are dalmatian mollies, they are just swimming around fearless and are about 2 cm now. 1 Ghost shrimp, and a bunch, countless really, of snails. I had a wisteria plant but the gold fish dug it up, the plants are floating around but never seem to die so I left them in for now. All of these smaller fish are less than 6 months in my tank. I put them in quarantine before integrating them with the big fish. They all get along fine and seem happy actually. In fact, one gourami was a scaredy cat until the rest were put in. it now comes out and stares at me and gets excited for feedings.
It's been really hot so the water has been slightly warmer than usual. Usually it's a bit colder but I don't know the exact temp. Roughly room temp, perhaps anywhere between 68 to 71 degrees as the weather changes too.
I don't use other equipment but I do use a special hose to siphon water and refill it by attaching it to the hose outside. Yes I fill it with tap water and use Easy Balance and water treatments.
The tank is near a window so it gets enough sun to promote algae growth which never gets out of control thanks to the snails. But I do have to clean the glass every other week or so. At night we turn on the florescent light for a few hours before bed.
I just did a 25% water change, I vacuumed the gravel. I do this about every other week but I had not done this in one month.
I feed the fish twice a day sometimes three. Tetra Tropical flakes.
So all of this is how I've been running my tank with no problems. About three months ago, my goldfish developed a bump. It looks like a bubble, like an abscess really. It does not seem to bother him but it grew and then stayed at the size it's at not approx half a cm in diameter. The fish doesn't rub or anything. Behavior did not change. Appetite did not change. At one point he was all red on his underbelly. it looked bloody. I thought it had to do with the bump so I removed him from the tank and put him in a quarantine tank. he got better after treating him but the bump never went away. I reintroduced him to the tank, no health problems since with him or the other but the bump persists.
I've looked and searched and I have found no cases like it. I find similar things but non look the same.
This fish is obviously healthy and won't die but has this cosmetic issue. I am almost tempted to fish him out and cut that thing open to drain it but obviously that may cause more harm than good.