18'' well need lots of time to feel at ease in a new tank, well maybe 2weeks tops. i got mine 15'' and i put it in my piranha tank and observe what the piranhas well think do, first they all were on the gravel while the aro is above, well i though no harm well come until one of the small piranha started nipping the tail so i decided to transfer it in my housemate tank which is full of goldfish and koi coz i didnt still have a tank for my aro since i only bought it coz the lfs was seeling it cheap. well now hes quite happy in his new tank.
