do you have to have substrate for large cichlids????? I was kinda wondering would it be ok if I put like a bag of beach pebble in my 150 which would obvioulsy not cover even close to the whole tank. It would kinda be like a bare bottom with some rocks spread out just to break up the bare bottom. Like do cichlids need substrate to dig??? one thing i'm worried about is that they may move the little rocks and when they spit them out they will fall on glass bottom would that be dangerous???