Your tank is cycling. That's the ammonia bacteria starting up.
Remember the first product of fish + food + water is ammonia, the bacteria that eats ammonia is everywhere, its one of the reasons we add chlorine to water supplies.
So your tank is starting a cycle, you've got a couple options.
If you want very little flow but to have biological function in the tank you can use an under-gravel filter with the lift pipes sticking right to the surface (no deflector heads) making a small rise in the water over the top of the pipe, you can move the pipes up and down to keep them at the surface. Under-gravel filters need maintenance requiring breaking down the tank every once and a while (yearly-ish).
Second option is the Tetra Whisper filter. At your local fish store OR at petco you'll find the whisper and small cylindrical sponges that go on the intake for Fluval Edge's filters. Stick that part way over the intake bottom of the whisper and then keep the top of the whisper filter at the waterline as best you can. This is more than enough filtration for a single betta in a three to five gallon tank. The foam acts as a pre-filter and bio media, the little carbon filter in the whisper does polish and chemical.
Your rock depth is too thick, no more than a half inch un-planted, two inches heavily planted or an inch with under-gravel filter plate. The rocks stagnate water flow and can produce sudden changes in water quality simply from your boy powering past them.
With a nice sponge to grow in, the slimy stuff should stop forming so fast on the tank walls.
Once you've got the little whisper set up you can buy a banana plant (about same price as a veil tail) to keep him company and put it opposite the filter flow. They're cute and sometimes float around.
Another but more expensive option is the zoo-med submersible filter 304, about $15.00 or more. (zoo med 318 is another option but they're about $32)
Last edited by Thunderloon; 02-11-2011 at 11:33 PM.