Just bought three harlequin rasboras on Monday (I already had three so I was bringing the shoal up to six). Added them to my tank in the usual way (float in bag for 20 minutes first etc.). I didn't quarantine them as the LFS quarantines them anyway, so I didn't think it would be necessary, although I'm beginning to this I might be wrong there! Yesterday one of the new ones died, and today one of the remaining two has it's mouth stuck open, as does one of the old ones. Any ideas? Water quality is fine, I've tested everything.
It gets worse... the older one which was affected has just died. On the plus side this gave me the opportunity to examine its mouth very closely - no sign of any inflammation or fungus. The inside of its mouth was white - not sure what colour the inside of a fish's mouth would usually be? But nothing "fluffy" like fungus as far as I could see.
Does the inside of the mouth look slightly "milky"? The mouth should be a pale pink colour and I suspect the Harlequins have columnaris which is a nasty bacterial disease. It often shows as cottony areas on the mouth or white patches like white mould on the skin, but it can sometimes be internal with less clear symptoms. The whiteness inside the mouth and their holding their mouths open along with rapid death does sound very much like it though.
Columnaris (or cotton-mouth) is caused by a bacterium called Flexibacter columnaris and is very contagious. It is difficult to treat successfully, but first line of treatment would be eSha 2000 to at least try and minimise the spread. Often anti-biotics are the only answer, but would involve a prescription from a Vet as they aren't available over the counter in the UK.
Do a large water change, remove any carbon from the filter and start treatment as soon as possible. FX for them.
Sorry it's been a while. Update on this situation: two more harlequins died the night I started this thread, but since then, no more have died or even shown symptoms. The remaining two harlequins are fine (apart from being a little lonely - I'll get them some friends soon) and all other fish remain unaffected. So it looks like whatever it was, it wasn't columnaris. I've treated the whole tank with eSHa 2000 just to be on the safe side though. I was lucky(ish) this time, but I've learned my lesson - I will quarantine all new fish from now on, not just rely on the fact that the LFS has quarantined them.