I have a problem with a few of my neon tetra. Some of them have a white growth around their mouths and Melafix/Pimafix is not curing it. Does anyone have any suggestions? I've attached a picture to this post. Cheers!
The best option would be to quarantine/hospitalise the affected neon tetra for treatment in a separate tank/container (e.g. a large Really Useful Box or other plastic container if you don't have a spare tank) with heater and spare filter (moving some of the mature media across into it), then return them to the main tank. [It's always handy having a spare heater and filter anyway, in the event of device failure, if you don't already have these - and then they can also be used for this purpose too.]
While you could remove the nerite snails, medicate the main tank and run carbon through the filter following treatment to remove any traces of medication, I think this is risky - there would be no guarantee that the snails wouldn't encounter some traces of copper.