Fish near bottom of tank
I have a well-established, planted 29 gallon community tank with primarily tetras (cardinal, red-eye, pristella), a single ghost cat, a single zebra danio, and 2 amano shrimp. This tank has been running for 7 years and all of the fish (and shrimp) in the tank have been in there for between 6 and 7 years. The fish are all healthy and I have never had any disease in the tank.
The issue is that over time, all the fish have gradually moved downward in the water column. At this point, all the fish spend all their time in about the bottom 3" of tank. They swim around and are generally as active as ever but they never move upward into the water column. I'm aware that tetras are middle/bottom dwellers for the most part but even they spent many years in the middle of the tank. Danios are known for being top dwellers and yet mine is down with all the other fish, in the bottom 3".
Any ideas why this is happening? How can I encourage the fish to use more of the available space? Why are they staying low? There is a good variety of aquatic plants available to them at all water levels. I have a single, 65W compact flourescent but it's not a brightness issue - I had extensive duckweed at one time and enough long val to completely cover the surface and it made no difference. I have a Rena canister filter with a spray bar aimed to flow across the surface about an inch down. There is not very much turbulence at any water level and I did throttle back flow for a few days last week but did not notice a difference in the fishes habits.
Thanks in advance for any thoughts or insight.