Question about JBJ nanocube

[Oilers]

I have a 28G nanocube for a few months now but the problem only occurs this week. The other day I noticed the back chamber (where I place the pumps) was running dry and the pumps are blowing air into the tank so I top it off. However, after I refilled the back chamber, I had water overflowing the tank and spilling onto the floor. So I took some water out of the tank to stop the water from overflowing but then the back chamber was running dry again and I couldn't get them to balance somehow. What am I doing wrong?