Why Has Patrick Got A Bulging Back End?

Hi all,
My new Betta Patrick has a bit of a bulging back end on both sides of his body. I know it's not normal, as neither of my other Bettas have this. Their bodies get thinner as you look from their head to their back end. Patrick seems happy, and is bubble nesting. Does anyone know what this is, and if it's contagious? He's sharing a tank with Sebastian, and they do have a thick plastic divider, but the water does pass through from one side to the other as the divider is not sealed to the tank. You can't see a real lot from the pic, but here it is.

I think I know what you mean and I've seen it in other bettas before, but I thought it just had something to do with the strain. Now you got me all curious, lol! Dousn't sound to bad though...

Hopefully it's normal for him then. He does seem healthy other than this bluging back end thing.

And BTW, Patrick is gorgeous! I LOVE his coloring!

I cant tell from the pic and am not entirely sure what you mean, but he could have a swollen stomach due to constipation

I thought of that too, but this bulging area is right at the very end of him, behind where the poo comes out, so I'm thinking it's probably not constipation.

Aww he is a cutie.. Lovely tail.. x