Hi, I bought a pair of Motoro stingrays a few months back, they were about 3-3.5 inches disk size , untill now they were growing normal at about 1/2 inch per month or slightly more but at 7-7.5 inch size now the growth seems to have stalled, is this normal or there is some problem with my fish, they are in a tank 5'x2'x2' and i am feeding them hikari sinking food.