Shrimp/Worm like organisms in my fish tank?

I have a 29 gallon tank with a large black moor goldfish in it. There are no plants. Just rocks at the bottom and a big pirate ship. We planned on plants, just haven’t got there yet. We used to have him in a 10g tank. We’re had this tank for about a month. We just bought a couple other fish today, and before we put them in to get used to the water, we did a partial water change. Well, after we did that, my husband and I started seeing these shrimp/worm-like looking things swimming around in there. They are small, yet visible to the naked eye. They remind me of tiny shrimp. Kind of wormy too tho. They seem to be clear and brown. Any suggestions what they could be? Are they harmful to my fish? Should I get rid of them? If so, how? Thanks so much for all your answers!
I think we might have figured out what it was. We just took the lid off of the tank to add some aquarium salt, and what looked like baby mosquitos were underneath the light. I think that’s what is swimming in the tank. Sounds kind of gross, I know. We just cleaned them off and we’ll check if often. We have 1 normal size window in the room and 2 smaller ones. 1/4 circle shape. They don’t have screens, so when we opened them, that must have been when they came in.

Thanks so much for the answers!!