I'd say forget the tumour!
It looks too broad, it's more like a wart sized lump on a human, or maybe one of them spots that seem to grow like, whoa man that's a big spot! Why me! The ones that make you plod about like this. . .
that type of spot.
I assumed the chaser to be male, normal activity for most if not all fish. Male chases female. Plus the more slender body shape, muscular you said, good visual indicator. Lack of tubercles? 50:50. Tubercles guarantees its a male. (Goldfish)
Lack of faeces, see my previous post. Possibly my first or second post. Key indicator, red swelling ties in with a post I was concerned with earlier, symptoms now similar to camallanus, killed my fist botia almorhae, B. lohachata (old name) maybe 7 years ago.
I can't believe you've never seen a glass catfish! I love these fish just for the fact they are see through, when they eat is why I can't get rid of them. When they die they turn cloudy like milky, some people suggest something in the blood makes them see through. Just magic with fins on.
Research, Cestoda 1500+ nematodes 10,000+ acanthocephala 400+
These are all considered worms.
Figures above are accurate, I made doubly sure and checked my references. It is impossible to know them all. Some are able to locate within the body cavity (lop sided) see, schistocephalus and ligula