Help needed from the betta people - real bad fish day

My son's betta seems to be very sick. I hasten to say he isn't in the same
tank as the barb in my earlier post.

He didn't turn up for tea tonight so I did a search. Found him at the bottom
of the tank but still alive. He's not swimming right and seems to be
bloated. Have checked the water quality and everything is as normal - 0
ammonia, 0 nitrite, very low nitrate and a pH of 7.5. He was in a 30gall
(UK) tank with two Pearl Gouramis, 2 Glowlight Tetras and 7 Harlequins.

I've moved him into the 5gall (UK) "fry" tank in with some baby platys and
Mollies....he's still at the bottom.

Any suggestions/help greatly appreciated. I'm not sure that he will last the
night the way he is right now....

Thanks
Gill