Burrfish with a swollen anus (photo)

[Diana]

Hi all, we have a burrfish that has been listless for the past couple of days. He is still eating, and today I discovered the area around his anus is quite swollen and the anus itself is open really wide:



I did a little research and there are two possible scenarios: one is constipation and the other is parasites. He is fed a diet of primarily brine shrimp (he'll eat a whole cube once per day), which is not very high in fiber and I'm going to get him eating some shell-on prawns and maybe nori if he'll take it, seeing if that will help. Has anyone had any experience with constipated fish or a swollen anus?

Heh, what a day, chasing a fish around trying to get a close up photo of its anus. I apologize for the number of times in this post that I have used the word anus.

Thanks for any help,
-Diana