Thanks everybody for your replies.
JanS, I managed to keep the silvers from the time they were little. They grew up and started mowing my plants : a potamogeton was cleared of the aquarium over night. I returned them to the lfs.
Jeff Kropp, the reason I'm turning it upside down is my un-inert gravel and not so good substrate I had from the start. It's not so favorable but I have no choice.