I set up a 48 gallon tank with 4 ranchu in it in early 1996. These fish have had minor health problems chronically since I put them in it. I considered each new problem a separate event, chalking it up to the "delicateness" of fancy goldfish. As is often the case, this kind of conclusion can be self-serving. The reality is that a lot of different factors can all cumulatively impact the health of the fish. It took me 6 months to figure that out. This is the tank with my home-made wet-dry filter in it. While I don't think that the filter is a bad idea, I do feel I may have cut too many corners over all.
Things came to a head in mid-1996 when all 4 fish contracted fin rot. Previous I had seen red veiny areas on the scales that disappeared within hours of water changes and pimples on some of the fishes' undersides that came and went. All the information I read about possible causes of these symptoms indicated that these problems were likely secondary results of some stressful environmental factor.
So I took an inventory of possible problems with my setup. From most likely to least:
This is a pretty horrifying list to my mind, in terms of taking risks. But it is the result of a lot of individual decisions that all seemed reasonable at the time. Just looking at it probabilistically, if I do 6 things that each carry a 10% risk of causing a problem, I have a better than 50% chance that something will go wrong. So this makes me think that a philosophy of trying to minimize the risk in each decision might be a wise idea. It also points out the importance of periodically checking the condition of the assorted aquarium apparatus.
Anyway, I've addressed each of these problems, except the last, which I have been able to discount. The fish seemed quite healthy, no pimples or fin rot, after all these changes were made. At the end of 1996, the fish are doing well, though the tailspotting condition discussed elsewhere persists. I have pretty much concluded that this is viral; it is endemic in the original population the fish came from, and doesn't seem to particularly harm them.