Again we have a pest that's not truly an enemy. On rare occasions leeches might attach themselves to fish, but this is the exception instead of the rule.
There are numerous species of leeches, far too numerous to enumerate here, but anything that crawls with a flattish body could be assumed to be a leech. Some species have a sticking disc at every end which enables them to crawl like a caterpillar, looping and extending the body.
They're very hardy, being able to withstand the action of chemicals to a greater extent than the plants, so it'll be obvious that treatment chemically isn't suitable. Raw meat suspended in the aquarium as for flat worms is about the most efficient technique.