Re: harlequins with mouths stuck open
#6
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.