Erika- I think that's a Mayfly larva. I haven't had very good luck raising them. They might do well on a diet of Cyclops if you have any (seriously, I think that's what they eat.) Fish eat Mayfly larvae when they get the chance and the larvae aren't too big. The bruise on the side of the rainbowfish is a cause for concern however. If it increases in size you might suspect a Mycobacterium infection. Clean fresh water and low carbohydrate food would be in order. =steev