Have had 5 guppies die in 2 weeks. My initial thought was that it was after the introduction of a guppy with a white flash on its side. To cut a long story short - the guppies' symptoms have been tiny fluffy like ball on their tails which have spread mostly on the tail, then killed them within 4 or 5 days OR finrot on the tails which have not responded to treatment. I've been treating with Pimafix and now also with Myxazin.
Tropical Fish Forever forum concluded that there's TB in my tank- especially because of the fact that the last guppy developed a curved spine- over a periond of about 3 weeks, gradually lost his appetite, faded in colour in the last 2 days then died. I was curious about your description of a gradual curvature. Would you call 3 weeks gradual? I'm in denial about Tb I think...
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Fish tuberculosis sometimes results in a gradual development of the curvature of the spine. I have seen this often in zebra danios but also in a golden orfe and a few other species. A fish with this condition (fish tuberculosis with a bent spine) is often also anorexic.
The fluff balls on the tails was fungus. It sounds like they had fungus and fin rot. I think that is what killed them. You should try a stronger antifungal and antibiotic like MarOxy and Maracyn by Mardel respectively. I'm not familiar with Myxazin. PimaFix is supposed to be all natural and safe and fight fungus but if it's bad, it may not be curative.
The symptoms of weight loss, color loss, and bent spine can be related to fish tuberculosis. I can't say how long it took my danios to go from unbent to bent since I had so many at that time. I think I would just notice it one day. So, becoming bent in 3 weeks would be possible with fish TB. That said, extreme illness such as the fungal and bacterial attacks that your guppies were undergoing certainly could cause the weight loss and color loss. I'm not as sure about the bent spine. I don't know that anyone has done a study on exact causes of bent spine. It might have occurred for a reason other than TB. If I had to guess, I would say that your fish developed fungus and fin rot from the new sick guppy that you added, and that fish TB was not the cause of their death. Whether or not they do have it or not, I really don't know. Some people think it's very common, with many fish carrying it, only to get sick from it if something else weakens the fish. Other people think if fish have it, you'd know. I do know that I have some fish that appear healthy but do carry fish TB such that fish put in with them come down with it eventually. I hope your fish don't have TB because it's really not curable. My total guess would be that they do not have it. Let's hope I'm right!