Do you still have the Lvl adjusting thing on the motor?
I just put an AC 20 on my 20g last night.
One of the first things I done was, break this off.
If you haven't cut it, to just the right length. It could be causing a weird load on the motor, if it and the adjustable on on the main body arn't perfect.
I just set the adjustable one on the main body to the second step. Thats usually worked for me. (It did again)
Put the sponge in.
The carbon bag.
Put it on the tank, with the intake in the UG tube. Which I cut to fit about a 1/4" below the AC body.
Filled the AC and turnt it on. Nice and quiet, as soon as the air cleared out. Going to have to do a DIY for the water comming out. To get rid of the water fall bubble/gurgle noise. It seems like the smaller the filter the more annoying the out flow is.
Then...
Woke up this morning.
And motor wasn't running. And warm.
Took it off. Plugged it back in. Smacked it. And good to go again.
Not sure what happened. But one possibility, would be. An air bubble came up through the UG and it got air locked(something like that). The got hot and quit?
I've had probably every size they make and very few problems.