"what's on my fish" ?

Roundworm (Trichostronglus ova)

Roundworms are very similar to
Camallanus worms These nematodes
produce the same symptoms as
Camallanus, but they don’t protrude out the anus of the fish.

These are thus rarely diagnosed in fish. These can be suspected if one is getting a lot of unexplained deaths.

Roundworms tend to hit livebearers especially hard for some unexplained reason. If only the livebearers in a tank are dying, suspect Roundworms.

Guppies seem to be especially hard hit.
Roundworms may well be more common than camallanus. It is often found in necropsies of cichlids which have shown such symptoms as bloat, white poop and not eating.

It thus imitates Hexamita.
Roundworms are unlike camallanus in that they produce eggs, not live larvae.

The best way to diagnose Roundworms is to look at the faeces of the fish with a microscope.

If there are barrel-shaped eggs in the faeces they have Roundworms.

It is only considered a virulent and potentially deadly problem when there are a lot of eggs in the faeces.

These eggs are found in small quantities in most fish in the tropical fish trade.

It is a subacute pathogen until something triggers it to become virulent.

Often slightly cloudy or “dull” water is found when fish start dying from Roundworms. The life cycle of Roundworms is relatively long. The eggs and larvae can exist outside a fish for months and the worms take three to four weeks to mature once inside the fish.