Tapeworm in Koi are nearly always bothriocephalus acheilognathi. They can be a problem mainly from planted semi wildlife koi ponds or bought in fish from raising ponds or mud ponds.

Many tape worm species have other host cycles e.g. birds, pond snails, etc. but are not always detachable.

The fish loses weight like hexamita. Mostly only a single fish is infected, it's very rare to have more than one at time, but I have seen four in one pond.

It can be treated. Tub the fish, wait for faeces, check for spores. If none are found treat for tape worm just in case. This sounds very haphazard, but to be sure, you would have to open the fish up.

Most tapeworm is sadly only found upon postmortems, where parasite treatments, used via a wrong diagnosis, have been given with a view to fish's condition or supposed starvation, which in a koi is nearly impossible when kept in a pond. I have seen the latter in a tank.

Symptoms can mimic hexamita, fish tuberculosis.

When a fish has carried the worm over a long period, the fish will look in a poor state. Very thin like a tadpole, all head, very thin body, with eyes sunk. It finds swimming hard and will eat if it can get to food. These are symptoms of TB as well, so it may not be a worm. But at this stage, recovery is only at 10%. The fish has lost most of its digestive tract to anorexic intestine.

Thankfully, it's not a common problem.