So 1 of your fish has a bent spine and 1 has mouth rot/cotton wool disease?
While a bent spine is a symptom of dropsy its not the only diagnosis. Why are you presuming its dropsy? Any other symptoms? Is the fish active and eating? If so its a bit extreme to get rid of it.
The mouth rot/cotton wool. Have you tried any treatments? I notice you are UK same as me, so some of the treatments that you might be recommended arent available here without vets prescription. I use Esha2000 to treat those kinds of ailments with some success. It also claims to treat dropsy.
Clean water and a suitable living environment is an important thing to get right. Could you give some more details on the tank. Size, temperature, filtration, water parameters (pH, ammonia, nitrite, nitrate), water change schedule, how old is the tank, plants, other fish, anything you can think of.