I've heard rumors Walmart is starting to remove the fish departments, and that they are all supposed to be gone in 2011. Just a rumor I guess but I'm hopeful.
I think the best way to tackle Walmart's care methods is a well written complaint straight to corporate. Boy did my letter light a fire under my local manager's ass..guy was blabbering on my answering machine apologizing, and yes things have improved. You don't want to be offensive or anything...just the facts about what you've found a problem with. Doubt they will care about things like cycling, but pointing out the dead/diseased fish and misinformation on the cards will help.
Talk with your money as well. I can't tell you how many times I've read about people complaining about Walmart...and then they go and buy/"rescue" fish there. If you keep buying their fish, there will be another fish there to replace it in a few days...you perpetuate the cycle. If there is no market for that particular product...they will either do away with it, or figure out a way to entice people back (better care standards, etc).
Just my two cents.