Why are my rasboras getting fungus?

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Gypsum

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One of my harlequin rasboras has fungus on its tail. It's the third rasbora to get this condition. The first one was three months ago, when we first got the fish, and we noticed it on one fish immediately after we got home from the fish shop, and took that fish straight back, writing it off as a dodgy fish or a netting injury. The second one was three weeks ago. We removed the fish to a quarantine tank and treated it with anti-fungus medication containing malachite green. The fungus cleared up after a few days, but the fish died shortly after reintroduction to the main tank. Now another rasbora has it, but no other species in the tank has ever shown any signs of any pathology. We do 50-60% water changes every week, and water parameters all copacetic (Ammonia, 0; nitrites, 0, nitrates, 0; PH 6.6)
rasbora.jpg
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Could it be a weak stock of rasboras?

Any treatment recommendations, and should we treat the whole tank this time or quarantine the fish?
 
Start with 50 percent water changes daily
 
Or even 75%
 
Someone like @Colin_T or @Deanasue will have a better grasp of this, but I do not think that is fungus. Before you start using medications, pin down the problem as best you can, the aforementioned members will help.

Malachite Green is not a safe medication to be using especially on soft water fish species. It may or may not have dealt with whatever this is last time; sometimes things like this just go away. But if the fish then died, it could have been the malachite green on top of whatever else.

Any substances added to the tank water will get inside the fish via osmosis. This causes additional stress at the very least, which further weakens the fish. If the additive is absolutely necessary, and is the safest and most effective, it is worth the additional risk.

Partial water changes (70%+ of the tank volume) can really help so often. Sometimes this alone can deal with issues.
 
Or even 75%
Is it fuzzy? I would start with daily water changes of 75% for 14 days. Cherry rasboras are pretty salt tolerant so I would add one tablespoon of dissolved aquarium salt for every 5 gallons of water on initial dose. After initial dose, do 3/4 tablespoon of dissolved aquarium salt For every 5 gallons with every 75% daily water change. See how he does after several days. If improvement, continue for rest of 14 day treatment. If not, get back to us so we can look at meds.
 
Since the OP lives in Scotland, the choice of meds will be somewhat limited, I'm afraid
 
Thanks. The rasboras have been in the hospital tank for about a week and we did attempt an anti-fungal treatment, which hasn't worked. I'll try the suggested salt and water changes.
 
Deanasue's queston about the fuzziness has not been answered. i still do not think this is a fungus; from the photo I have seen this in the past, and it is more like a growth, but I am going from the photo.
 
Yes, it looks fuzzy.

Fungus is always secondary, meaning that it occurs because of some other initial problem, such as a scrape, bite, or whatever. And now that I've looked again at the photo (still can't really see if this is fungus or not) I see a roundish red area on the caudal peduncle. This is not showing on the other rasboras in the photo, so it is the underlying issue that should (maybe) need treating.
 
Aye, I figured it had an underlying issue, but I don't know what that is. I'd reckoned that the first rasbora who had fungus-like growths on it had suffered some kind of injury. But now that this one has it, I have no idea what's going on them. No issues on any other fish.
 
I treated the rasboras as per the instructions above, to no avail. Then I tried another round of Myxazin, which didn't work. The fish deteriorated, looking pale in colour, suffering from difficulty swimming and rapid respiration, and I euthanised it with clove oil. On post mortem, I saw it had a deep lesion at the base of its tail where the fungus-like growth had been. As the other rasboras looked healthy (and at this point, had been in quarantine for a month), I returned them to the main tank.

Only this morning, I discovered that one of them had more white lesions at the base of its tail. Back into quarantine they go. Any ideas?
 

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