So I did some research and set a terrarium in a 20 gallon fish tank. I purchased African violets and a couple of green nonflowering plants a vines. I put a layer of gravel, then carbon, then sphagnum moss, then the soil.

Today I wake up and look at the tank and I see two slugs, I think. they were like a worm but flat and hat two little antennae sticking out of their "head". Are these bad? What can I do to get rid of them?