They are itty bitty tiny little green bugs, right? I had them once during the summer. DONT open your windows, this is the best way to prevent them. Especially dont open them at night because the light attracts the bugs to the water. Also, I filled mytank with really hot water. Mix it in the gravel. The larvae of these bugs (whatever they are) are in the gravel, and come to the top when mature. The hot water did it for me, but you have to make sure they dont reinfest.