Ok, here I am, mister homoeopathy. May I suggest trying to keep them in different conditions? Clean the tank out and perhaps add a teaspoon of table salt (or cooking or whatever, it's not too important - iodized doesn't matter) per 5 litres / 9 pints of tank water and see how the they get on for a few days? It's amazing how helpful such a simple approach can be. I've never lost an aquatic Salamandrid to disease and all I've ever treated them with was simple remedies like this.
Now having said that let me add a disclaimer - if your newt is suffering from something that isn't just an external infection (i.e. it has septicemia) then you may need to resort to more drastic measures but usually the most effective method of administration of antibiotics is by needle. Try the salt for a few days though, just don't add more than I suggested or you will do more harm than good.
Oh and I've also found that it's often easier to help aquatic newts with wounds to recover by removing them to a "clean" terrestrial environment.