Since starting my pond up I have noticed some behaviors that I have never seen before while keeping fish in a tank. Basically, when it gets dark out there all the fish go to the bottom and just sit on there bottom fins, they are still exhibiting full color, and they do not seem to be "breathing" heavily or otherwise labored in any way. When I go to feed them again in the morning they are fully active and back to normal. Should I be worried, is this something that fish do when they are in the wild and are just now exhibiting those behaviors because they are in a large body of water? Thanks in advance guys and gals!