from my experience, monos mostly stay at the bottom to middle of the tank but more often somewhere in the middle. if in a community tank, they will tend to linger at a chosen spot (usually somewhere in the back,out of the limelight) which they will protect. so far, my monos hv been pretty non-aggressive cz i chose them specifically when buying - look for the non-aggressive ones at the lfs tank lar! it's also better if u buy them small.

but they shd never go to the top unless it's feeding time cz then sth is wrong. the other way to tell if sth is wrong is if they start to look as if they're turning black, ie they're no longer silver but hv a black shadow cast over them. this is usually a sign of stress (which u might hv noticed when u first brought them home and put them in the tank) and if it does not go away for more than a couple of hrs or so, then do start to get worried.

the "skating" thing, i noticed when i tried to catch some from the lfs tank to purchase. otherwise, i donno if they do this.

generally, pricing for monos ranges from rm7-13. got my sabae for rm7 at irene aquarium and my arg for rm8 at A1. bigger lfs like pets wonderland, etc tend to price them higher.