I have 3 rb piranha. They're growing super quick and coloring nicely but I've had this problem since I got them. If whatever I feed them doesn't fall almost in front of their face they won't eat it. They also don't eat off the top of the water or the bottom. How normal is this? They know when I put food in the water because they start swimming around and looking for it but if it hits the bottom it will sit there until I scoop it out.