Snail? Half a clam? What is this?

I saw this critter in my tank maybe 4 weeks ago, they barely move around, and they are very hard to get off the glass, they stick very well. I had one small guy stay on the same leaf for at least a week, did a good job on the algae. I noticed a bigger one down in the substrate a few days ago, and today he has popped up on the glass, again, barely moving around.

 

I’m interested to see how big they can get. My water is low on the KH side under 1 degree. I think they came from my pond when I brought a bunch of pond plants into my tank, or maybe a lake in haliburton when I tried growing Canadian pondweed.....life........finds a way.

 

The limpets won’t hurt the shrimp, they are like snails, you can leave them alone. You said you had planaria? The planaria will harm other livestock, in my experience, I’ve seen planaria kill fish/shrimp, granted I had a ton of planaria in that tank.

 

That’s what those were! I had those in my new tank. I was so paranoid they might be younger planaria, I started vacuuming these guys up. I wasn’t sure because they looked a bit like them, but no triangle head.