Angelfish Anatomy

Does anyone have any ideas about my problem below or knowledge of internal angelfish anatomy?

Symptoms: Fish had been living happily alongside 9 of its brothers and sisters, though this one had quite slow growth development (1.5 - 2.0 inch) and was probably one of the weakest of the group. It began circling around yesterday, head over bottom and docile enough for me to pick it out of the water to transfer into another isolated tank of the same water. No other external signs, externally the fish was perfect. The other tank inhabitants displaying normal behavior. I treated for anti-slime and velvet, this fish was on its last legs.

Water parameters: Temp 27. pH 6.5, Ammonia 0, Nitrite 0, Nitrate 40ppm (and lowering).

Not for the faint hearted: Not unexpectedly it was found dead this morning. I inspected its gills, these looked healthy enough a bit purple (no blood flow) with no obvious signs of parasites. I do not have a microscope, so all my observations are naked eye. Working on the quarter (6 to 9 on a clock) Internally the organs were quite hard to tell what was what. Near the ventrals was a 5mm round milky coloured sack, which seemed to contain milky puss and lighter blobs, on opening the stench was like a very very bad throat infection.

I think I identified the heart (a small red blob), but in addition but separate to the heart there was also another larger red blob 2-3mm which contained around 15 black specs. This seemed to be attached to another organ but I do not know what that was. Any ideas?

Anyone here an autopsy expert? or know the best incisions / methods to make when trying to find out what killed your fish?

Sorry, dissection has never been my strong point. Had to dissect a dogfish for my A level biology, my teacher described it as the worse dissection he'd ever seen. I even struggle carving the Sunday roast!
If you google "fish anatomy" you'll find lots of diagrams which might help you!