I just noticed that a few days ago that i'm hearing like somebody is tapping on my aquarium, but no one is near it...I just spotted today that my clown loaches are the ones that are doing it and they're really doing it hard( I could hear the knocking noise across the room)...Is this normal cause i'm scared that they could actually crack the glass and I have to mop 75 gallons of water in my living room.....is there something wrong with them???