I have a 20 gallon tank that isn't shallow. I have just purchased a red belly piranah that is 3 inches in length. I have had it for 2 weeks and it never really moves and will only eat if the feeder fish moves right infront of its mouth. My temperature of the tank it set at 78- 80 and it has alot of oxygen because of the numerous bubblers i have set up. I have a bio wheel filter. I also have put in three logs and are growing 4 amazon sword plants. Any tips on what I can do to increase the piranahs movement such as dimming the light, darkening the water etc.