I had a pregnant ghost shrimp. I put her in a ten gallon, let her give birth, and then took her out. I fed the little guys but they're dead two weeks later, it would seem, all at the bottom of the tank. :-( However, running around them are hundreds upon hundreds of these little critters. No larger than the tip of a pin. I can see them with my flashlight. I'm concerned about putting the plant in that tank back in the main tank--what are these things?!?