rainbow swimming strangely

Hi,
I have an 80 litre community tank. It has an overhead filter, plus airstone.
There are one large comet, 3 swordtails, two SAEs and two rainbows in the
tank. The larger rainbow has always had a tendency to ich, which only
affects it, and which we treat immediately symptoms appear. The symptoms go,
but then reappear a few weeks later. The other fish never show any symptoms.

Yesterday one SAE was on its back. Water was changed a weeks ago, and all
parameters fine - nitries 0 PH neautral etc. Yesterday the water was acidic,
ammonia .5-1. DId water change, added bicarb for acidity. SAE much better,
but noticed rainbow was swimming with its body slanted upwards, and it is
almost frenetic - as tho if it stops it will sink to the bottom. It is
wiggling its body a lot, ie, almost like an eel, not a fish, to move thru
the water. I added ich treatment and an intibiotic, just in case.
Today ammonia has dropepd, PH neutral, all other fish are fine, including
the SAE, but this fish is no better. I have moved him to a floating breeding
tank, as I thought he was wearing himself out, and he is much calmer in it.
I've down the physical, and according to that he should be well. The only
sympton besides the frantic swimming and his position is that he looks
skinny - as tho his sides are shrinking in, and he took food flakes but spat
them out.
My husband thinks its the swim bladder, anyone seen this before or have any
ideas? We've had this fish for 18 months

TIA for nay help

Judy