curved body

Hi, I have had a tank going since June last year and all has been well. Recently I have noticed 2 of my sunset platys acting strange. At first just one but now both of them appeared to be curved and are swimming round in circles or sitting at the bottom of the tank.Any ideas what is wrong?

I ought to be settling down now with my new fish after 3 months, and in fact they're all doing very nicely in general. I am however keeping an eye on my blue dwarf gourami's change in behaviour. I've been reading around elsewhere before posting here, to try and get an idea of whether he is...