good point about the peas. I have goldfish that get constipated regularly and I give them peeled peas and it sorts them out.
@Micky I never heard of epsom salts in the water. have you tried it. Does it cause things to go too much the other way.

I dont want brow/green coloured water if possible.
@NCDUB . It sounds like swim bladder trouble which seems to go hand in hand with constipatiion as micky said. When my goldfish is constipated he swims slightly sideways but the peas sort him out. I also think that the sooner you get your filter problem sorted out the better. This should make a huge difference. Hope you fish feels better soon.