I just got a 30 gal community tank with a few bala sharks in it. Iwent to bed with three in the tank and when I woke up there were only two balas. I looked all over the tank and in the filter. I have a full tight fitting lid so it couldn't have jumped out. It is a tank with mostly semi aggressive fish so the only conclusion I've come to is that he was eaten but there is no trace of any remains. I got over that loss but tonight, when I turned off the flourescent light in the hood, one of the balas went into a spasm. He swam all over then went belly up. Can someone please give me an explanation for either of these losses? Am I going to lose a fish every time I turn the light off?