Long story:
I set up the tank on Thursday in preparation for 6 L260 I would be receiving on Friday.
Volume: 30 gallons
Dimensions: 36" x 12" x 16"
Filter: Penguin 150
Temperature: 82 F
Also: I have a Maxi-Jet 600 Powerhead at the bottom of the tank pointed down the length of the aquarium. There is no substrate and no background.
Picture:

I received the fish on Friday and acclimated them over an hour or so. There are 6 fish, all approximately 1" long including the tail. I released the fish into the tank, and also added a few rocks (slate) I collected and rinsed off for them to hide under. Tank lights have been off for the most part except for when I took some pictures. After being in the tank, they mostly stayed on or under a rock that was closest to the powerhead. I tried feeding some earthworm sticks but they do not respond. Everything seems normal for newly bought plecs so far.
Come this morning (Saturday), I find that the fish are all collected at the top in various corners of the tank. They also hung out behind the tube of the filter or behind the wire of the heater. They would sometimes move between spots. Coloration appears to be normal. I was unable to tell how fast they were breathing. My immediate thought is that oxygenation may not be high enough, so I put an air stone in the tank attached to a Whisper air pump. This appeared to have no effect. My next thought is they're trying to "escape" due to water quality. I test the water and get these parameters:
pH: 7
NH3: 0 ppm
NO2: 0 ppm
NO3: 0 ppm
KH: <3 degrees
GH/KH: <50 ppm
All test kits are API dropper test kits. I believe nitrogenous wastes to be low since the tank is pretty new. For both KH and the GH/KH test, I only needed 3 drops to make the color change. I had e-mailed the local water company and go this response: "The latest data we have on hardness and conductivity is from the 29th of April 2009. Hardness was 40 ppm as CaCo3. Conductivity was 119uMHO/CM. We are required to test these once a year when we do our IOCs."
I wasn't sure if my eyes were fooling me with the ammonia reading, so just in case ammonia was higher than I thought I did a 50% water change, this time filling the water up only so high so that the water outlet on the filter splashed more as it entered the tank; before the water was slightly above the edge of the outlet. I also have treated Seachem Stability, a bacterial additive. The fish haven't really moved, but I think they are slightly lower into the water than they were before.
My last possible thought is that they simply don't like the bottom for some reason, but I'm not sure what I'm going to do to make them like it better. This is the first time I've placed a pump near the bottom of the tank, and the first time I've kept plecs in a bottomless tank.
Help me out, guys, please!