Fish Health In My Tank

Connected with Think Fish Keepers Daily News thread post #1733 and subsequent posts from that thread #1742-1750 which were merged with this thread into posts #52-60, another microdevario kubotai / neon green rasbora (MDK/NGR) in hospital tank now with eSHa 2000...   

Last week, the day before moving Hump-backed NGR into it, I noticed another NGR had developed a feature - a dark spot on its flank, probably near its intestines. The following day, it had developed two smaller dark spots in the same position on the other flank. I've been monitoring the situation and thought the larger spot was worsening/deepening. Strangely, that fish was missing this morning - but then re-appeared from wherever on the 5th attempt of counting plus opening and shutting the lid.

This evening, I've just discovered that fish breathing/gasping/drinking rapidly from the surface and the dark spot is a definite ulcer, right through to the white of the internal organ - precisely what and in the same location as one of the two female tetras had recurringly for 18 months before succumbing to it and possibly similar to Bent Spine Harley's although his is less deep, further up his flank, seems to be cause of his curved spine, and he seems to be living with it better than I'd anticipated.  I have little hope here due to the fast breathing/gasping/drinking and the track record I've had with these fish (5/12 left including him and they're only 0.75 year old).

Any thoughts on whether I ought to be trying a medication for parasites instead? I've never seen anything protruding from any of the ulcers in the aforementioned fish.

[...and Mr FCMF wants me to get rid of the fishtank once the current stock have depleted as he insists "fishkeeping's too much hassle"...  ]

Update (following morning): Fish is lifeless on bottom of hospital tank , a particular shame as he seemed to be doing well in it throughout the evening. That leaves only one fish left from that particular batch of 8, one with a forked tail and who has been fine until now but is hiding on his own at the bottom of the tank and didn't bother to have breakfast...

Thanks, @Littlefish. The final fish in that original batch of 8 (Forktail) has been taking to playing hide'n'seek in the mornings, something the previous fish did just before dying. I check the water surface every morning for any overnight fatalities, drop in the fish food, try counting and re-counting several times, then get out a torch to scan the bottom of the tank for any bodies, at which point this particular fish suddenly appears.  Only if a week goes past without a fatality, might I settle a little...