Ok so while looking in my 5g I noticed something swimming quickly through the tank. Racing around was a gammarus shrimp or scud. It's a freshwater crustacean. I think I figured out how it got in there. I have a turkey baster that I use on my tanks to suck up any unwanted items. I have scuds in all of my FW shrimp tanks. I like them & they're fun to watch. They don't harm the tank & feed and dead & decaying plant material. Great live food souce for fish. I believe it was in the turkey baster when I used it to remove some of the baby MTS snails in the tank. I will try to remove it when I get a chance. There are salt water versions of scuds as well.
Algae In Space liked this