Hey there. I don't know what the temps are in your area. The cure doesn't always seem to work under 50oF according to the info I got. Water fleas are also called fish lice and can be a very serious problem for comets and koi. The leeches seem to be a gastronomic treat for the fishies though I would be willing to deny them this delicacy without a qualm but I don't see any site that recommends a particular treatment for the leeches.
The medications I had the best luck with are Dimlin and Anchors Away. One of the aquarium stores here uses Clout. The usual information is for treating the fish and the pond water but it should work on the plants as well. Sandy

If by water fleas, you mean gammarus species, they do not harm fish and fish like them as food when they can catch them. There are lots of little similar critters that like to live in your pond and do not harm fish. Ditto with the small flat worms. Don't treat unless you know you have parasites.

Here is a link that might be useful: Gammarus

if the "fleas" are water lice, the will die by themselves if denied a host. they will be gone in a month. fish lice look like horseshoe crabs or trilobites (google fish lice for a picture). don't know of any other parasite with legs. water fleas however are an good source of food for young/small fish.

fish love leaches and most other worms. i would check them to be sure they are nothing to worry about like flukes or anchor worms, but it sounds like little white worms which people actually culture to feed their goldfish.

post a picture, i am sure someone can ID them for you.

You can get rid of leeches by putting some raw chopped liver in burlap or j cloth over night. It should be full of them in the morning. You have to repeat till you get no more.