Fish dropping like flies

I'm having issues with a 120G community tank, yesterday everything was fine but this morning I found 1 of my lyre tail mollies sitting still on the bottom, I moved him to a hospital tank and he has started to recover. This afternoon I found a dalmatian Molly in the same tank swimming in what could best be described as a corkscrew motion. He was rotating round ( like a barrel roll ) as he moved forward, I moved him to another hospital tank but he is now lying motionless but still breathing, I think he will die soon.
Both fish were fine yesterday and behaving as normal. I am concerned for the rest of the tank, some fish I have had for years are in it. I reached for my test kit and found everything normal. I have dosed the tank with Sera baktopur ( I'm in the UK so stronger meds. aren't available ). Does anyone out there have an idea what disease I may be dealing with? Is there something else I could do?
There has been nothing added to the tank recently,. There are no visible signs of parasites or fungus.
The tank is fully cycled and has been for a couple of years. There is also a UV unit in the return pipe from the filter.
The tank gets a 30% water change each week.
Any ideas gratefully received
Thank you
How old are your mollies? Livebearers have a notoriously short lifespan. Somewhere between 2-3 years I believe.
Have you introduced anything new to the tank? Fish, decorations or plants?
  • Thread Starter
The lyretail is about a year old, the dalmatian about 18 months. If only 1 fish had been affected I may have considered it to be old age but the fact 2 fish got unwell within a few hours makes me suspect something more is wrong.
Nothing new has been added to the tank for months.

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