Cause of Death? Internal Organ Damage/Explosion?

[SkullV]

So I've had this Borb for a few months now and it has been doing great. A couple days ago it basically stopped eating but didn't have a mark on it. Still a perfect looking fish. Yesterday I watched it take its last breath and when I removed it found this:

http://i211.photobucket.com/albums/bb102/rockyrubicon/Fish%20Tank/40g%20Breeder/5D342BEA-B77B-4113-9A5E-DE50FEBE1144_zpsvctaf7ew.jpg

In an (weak) attempt to see what happened I applied a tiny bit of pressure to the site and blood/fluid seemed to excrete from the anus:

http://i211.photobucket.com/albums/bb102/rockyrubicon/Fish%20Tank/40g%20Breeder/4D23B7CA-2A25-44D8-91D1-33DAB1657D21_zpsolm692kg.jpg

Any thoughts on what happened to this fish? I would have guessed swim bladder but it has been perfectly happy for months. Second guess would be internal parasites, but it ate extremely well and never showed any signs of wasting. The only change in the tank over the last 2 weeks was a rise in Mg to 1900ppm to combat a small patch of bryopsis.

Any input would be greatly appreciated.

[krullshards]

Wow that's crazy. I've had fish get constipated and die before. Saddle Valentini to be exact, but it didn't really look like that.

The puffer also ate like a champ until one day when it stopped... Might he have eaten too much or something different than normal and gotten stopped up?

Did you feed anything different recently?

[SkullV]

Wow that's crazy. I've had fish get constipated and die before. Saddle Valentini to be exact, but it didn't really look like that.

The puffer also ate like a champ until one day when it stopped... Might he have eaten too much or something different than normal and gotten stopped up?

Did you feed anything different recently?

Nope, nothing. All it are was Larrys Fish Formula and PE mysis.