Community Tank (including loaches) with dropsy and ulcers - HELP!!

Dear all,

I am looking for some advice. I am a complete novice at fishkeeping and I have a serious problem in my tank. The tank has been up and running since the beginning of the year, I've kept a close eye on the water quality and it always seems fine. The tank is a juwel trigon 350litre, I have a huge piece of bogwood, a small piece of mapani (excuse spelling!) and some live plants (at the moment only fern-type plants, as the clown loaches and moonlight gouramis keep digging up the elodea and grassy plants). I also have a separate hospital tank, which only has gravel in it.
I've now lost all my dwarf neon gouramis and flame gouramis to disease, not sure of the disease, but they started to develop holes in their fins and then developed big red sores and then died. I treated both the tanks with Myxazin, but to no avail. I've also lost a couple of flame tetras to dropsy, which I used octozin - this seemed to work.
I have been told that these 2 treatments are the only ones I can use because I have clown loaches, but I don't think they work well!
I've also been told to strip out the tank, clean the bogwood, gravel and filters (I always hoover the gravel and bogwood when siphoning the water out), but am afraid that this will stress the fish who are already under stress of disease. I have thought about putting the loaches into the hospital tank so that I can treat the big tank with stronger treatments, but again I'm afraid this will stress the loaches.
I have just bought the following and wondered if these are safe for the loaches (naturally I follow all directions and dont mix treatments), the packaging says it's safe for all fish. Interpet 9: Anti internal bacteria, King British: revitaliser tonic, Melafix, and Doc Wellfish's aquarium salt. Can I use these?
The fish I have a tetras (neon, flame, albino glowlight, yellow phantom, rosy), silver shark, king tiger plec, moonlight gouramis and of course clown loaches. The shark, loaches and plec have not shown any signs of disease.

I would be very grateful if anyone can help me with these problems - I hate the fact that my pets are ill and I don't have a clue how to help them.