Jumbo Amphipods And A Leech

I recently added some plants collected locally to my 90 gal planted molly tank. Being in a hurry, I dumped 'em in without cleaning them as good as normally.
I noticed several different insect larvae and some really large amphipods in my tank afterwords. I've never seen such big ones in freshwater. They reminded me of the big ones that run around on the sand at the water's edge at the beach [ocean].
Anyone have an idea of what species they might be? I'm in northern massachusetts. Should I be worried about them harming my fish or eating my plants?
I also noticed a small leech. First time I ever had a leech in my tank. I noticed two cool things. First, the leech appeared to be desending from floating plants at the surface on the underwater equivelent of a spider web.
Second, I watched two different fish swim over and suck in the leech and immediately spit it out and dart off obviously distressed. They were gasping and doing the fishy version of spitting in disgust. The thing must have been toxic.
Any guesses on what it is? Any threat to my fish?

the leech appeared to be desending from floating plants at the surface on the underwater equivelent of a spider web.

slime. likely what the fish didn't like. They don't generally eat slugs either

I would remove it lest is starts sucking on one of your fish.

I would remove it lest is starts sucking on one of your fish.

Great. Now you tell me.
I was so facinated watching it I didn't think to grab it before in disapeared into the plants. Now I got about as much chance of finding it as finding a mouse in the jungle.
If it grabs onto a fish I'll just dump the fish. Hopefully it'll be on one of the half grown offspring and not my big beautiful male.

My experience is quite different... I found a leech the other day when cleaning up some ponds... pretty small, about 1.5 inches... I tossed him in a tank with a couple of banded sunfish... one came up, looked at the falling leech sort of sideways like a puppy... and ate it with obvious pleasure... looked at me, looked at the surface of the water, looked at me again...

Maybe different species of leech? Maybe sunfish are just pigs (even the little ones)

Great. Now you tell me.
I was so facinated watching it I didn't think to grab it before in disapeared into the plants. Now I got about as much chance of finding it as finding a mouse in the jungle.
If it grabs onto a fish I'll just dump the fish. Hopefully it'll be on one of the half grown offspring and not my big beautiful male.

I was surprised. I'd always heard fish ate leeches. I've even tied flies that were suposed to look like a leech.
Maybe only some species are unpalatable?
That's why I posted the question hoping an invert expert might clue me in.