Fish like little places to hide. You see comments all the time about how they like having side-ways flower pots or 4" PVC pipe bends to hide in.
I've noticed that my betta, which lives in one of those Anchor Hocking planter/fishbowls as seen in my November post on plumbing options loves to hang out in the roots.
I've also noticed that the minnows and bream I have in my backyard hug the bottom and sides of the tank.
Contemplating the joy my betta appears to derive from hiding in the roots and inspired by the Honey Hole Tree/Schrub design, I'm thinking of creating some kind of fish cover that would fit in a Rubbermaid stock tank. The Honey Hole designs are lovely weighted things sized for ponds of an acre of so--too large for my tanks. I've contacted them to see if I would be infringing on their intellectual property to make something out of trash can lid, tubing, and a rock...
The bad thing about fish cover would be if a fish decided it felt ill and swam into the cover to die. The good thing is I could 'inspect' under the cover if my water chemistry started going whack. But I think the fish will be a lot happier if they can have cover of some sort. Of course, they would also likely be happier if I could arrange for the weather to get warm and stay warm, but that I can't control.