Sick silver fantail (listless, fluttering fins)

Hi I'd be grateful for any help you could give me with an ailing fish.

He is a silver fantail about 3 inches long and has been with me for
about 2 months, fit and well untill the last few days. He has been
very listless, preferring to lay on or near the bottom and has started
to clamp (not as tightly as I have in very sick fish in the past) his
pectoral fins. Yesterday he took some food but today has totally
ignored it, he has started hiding behind a plastic plant in a way I
have never seen before and is also 'convulsing' in what looks to me
like a piscine version of an epileptic event.

Basic water parameters are all fine (ammonia and oxygen not tested but
oxygen is seemingly fine), his tank mate an oranda is well. I try and
mix up their diet with a combination of chopped earthworms, daphnia,
skinned peas and regular flakes.

I'm fond of him and on the off chance there is anything I can do to
increase his chances I'd be very happy to know it!
Many thanks,
Ed