Otinoculus With Grossly Swollen Abdomen. Infection Maybe?

I have never come across this before, I'm afraid. I don't know what to suggest. The blood streaked vent doesn't sound good and is definitely suggestive of an infection of some sort, though whether bacterial, viral or something else (protozoan perhaps?) it is impossible to say.
 
I will now keep an eye on my otos, when I can see them.

Watching with interest as I am picking up 6 Otos tomorrow.

Let us know how it goes Fiona, and good luck.

The affected fish have been the larger ones that we assumed were females and they had the same roundness displayed in that linked picture but like I said I did do a postmortem and there was nothing inside, no eggs and an empty bowel. From my limited experience, these fish don't gorge, even if there's an excess of food.

These otinoculus aren't new, they're ones we've had for about 2 years, the biggest were about 2 inches long and we know one was born in the tank.

I can't find the ill one at the moment but they do disappear and there's no ammonia in the tank, that said it could have died and been eaten.

After all the worry when we first had them and then finding a baby and the elation of that, this is a bit of a kick in the teeth to be honest.