Catfish Problem and Heter Problem

Folks, thanks for listening: 55 gal, Soft water pH 6.8, planted. 5
cardinals, 10 Black Neon tetras, 2 1" SAE, 1 Clown Pelco (Which I never see)
and one pair German blue Rams. 0 Ammonia, Nitrites, 5ppm Nitrates. (I've
tried keeping freshwater shrimp in the tank but they all disappear w/in a
day or so.)

Up 'till a few days ago my little Corydoras Julii would move all over
the aquarium, searching out food. (The tank has been established for about
5 months.) Now, he sits in the back not moving much. He seems fine, the
dorsal din is up and his gills are moving, but he doesn't come out much
anymore. Sometime I have to hunt to find him in the planted tank. Anyone
else seen this behavior? The only thing I've changed is that I've added a
Reva XP2 canister filter to the tank in combination to my Marineland 350
Biowheel. It really sets up a current! I was a bit worried about how much
flow the tank was getting but the neon's actually seem to like the current:
They school over to the inlet valve and 'ride the current'. By the way,
would this be too much current for discus, which I'm planning down the
road? I understand they prefer still water. But it sure keeps the tank
clean!

Also I purchased a Jager heater 5 months ago and it kept the water w/in
2 degrees of what I had set it at: 82F. Now it won't go over 80F even if I
have the dial maxed out. (I've checked the temp with two thermo's.) Dog of
a heater?

Thanks for any advise/help/comments. Please excuse this run on post!