I have 5 emerald cories in my tank (36 gallon bowfront). I used to have three and about 1 week ago I added two more. The two new ones are very active and always exploring the tank. One of the older ones (smaller of the old three) kinda jumps in every now and then and "plays" with the other two. Problem is my two big cories always sit in the shadows and are pretty much boring. Anyone know what the problem may be?