After posting the last bit I decided that treatment shouldn't wait so I managed to find a clear plastic container (which I scrubbed well with straight water) and have the sick fish floating in the tank. He can still see all his mates so may be less stressed as he dies :(  I added some antibiotic powder to some food and fed him then added the powder to his water at the 10mg/litre rate. He doesn't seem stressed by it so maybe I should go for the 20-100mg rate?? Yes it is a bit short on precision :))

I then added Melafix to the main tank as it was a safer option and may prevent any more infections until I have time to find out more about the oxytetracycline.

I've been using the oxytetracycline since 1996 for goat kids to prevent bloat would you believe. Despite a lot of effort on my vet's part, the cause of the bloat was never determined and when the last kid died screaming in his surgery in agony, he threw up his arms and said Here try this and handed me the oxytet. I've tried not using it as I'm not an advocate of indiscriminate use of antibiotics but it always ends in tragedy.

Tetracycline and promiscuity - ahhh there's the answer to my 2.8 kids per goat problem :)) I can tell you though that the fish with dropsy ain't too promiscuous at the moment. I've discovered I accidentally put a very young female in that tank too which I now can't remove so I hope she survives - I may have to have 2 floating dishes.

The bacteria may have arrived with the creek water or the snail that was in there for all of 30 minutes. The affected fish is one that Katie bought so I don't know how old he is but greater than 5 months. All the males are go, go, go all the time - maybe he just stressed himself out too much and now bacteria that came with the original fish purchases can shine through??