Can Snails Get Whitespot?

Snails can't get whitespot. But a snail that is in a tank with fish that have whitespot can get the parasite in the water on and inside its shell and transmit it that way. A bit like how we don't get foot and mouth disease but we can transmit it via the mud on our shoes, which is why farmers have to use disinfectant foot baths before leaving and entering an infected farm.

It is much more likely that the whitespot came from the shop. If there had been just one newly formed cyst make its way into your QT it would have lain there while the bug inside multiplied, then split to release the free swimming stage which would then have attached to the fish but taken a few days to become big enough to see. And because you had the heater set at usual fish temps, it took longer for this to happen than when we add medication, and at the same time increase the temp.

And there is evidence that stressed fish get it more easily and worse than non-stressed fish. Your cardinals had just been through the stressful time of being in a shop tank - gawked at by those huge things outside the tank, chased round by a net every time someone wanted to buy some, then caught themselves and put into a strange environment.
And I do wonder if this genus is also more susceptible than others. My related green neon tetras (they and cardinals are both Paracheirodons) always got whitespot worse than any other fish.

Hi Fiona

I moved my lone WCMM back into the main tank once I thought it was safe to do so as like your  rummynose he was all alone in the quarantine tank after the demise of the rest of his group.

He has now settled back into the main tank.  After a couple of false starts when he tried displaying to them he has managed to convince the glowlight danios he's one of them and has even got the hang of the nudging, nipping and jostling they do to sort out who's who in the group.

So fingers crossed your rummynose survives and gets the chance to settles in with the rest of your fish.

Anne

My water is liquid rock pretty much. There's a few of them with spots, they are soft and can be scrapped off.

The last rummynose died over night so the tank is now empty which raises another question.

I can't put the quarantine tank filter back in my main tank as its bound to contain infected water. I can drain the tank and clean that . I don't want to dump the plants I put in there. So how do I make sure the plants and filter are completely disease free?

I could move the plants,although it would have to be a cold tank

It depends what you mean by "cold", an unheated tank in a heated room would be OK, but not cold in the sense of "it's xxxxxxx cold outside today".