I did made total service on my ATL-1000 (which is almost the same as ATL-1500). I remove all bottles, main board, and evrything possible. In fact, I serviced it totaly. In plastic wires (for air pressure and for chemistry) You might find a lot of dirt. The most important thing is, that You set all plastic wires back, and they must have enough space for normal airflow. It happens, that some (because of air preasure) felt off the bottles. Use silicon glue to set and fix them back. You should be very careful by removing thermostat in bottle number one, because it is made from glass, and it is easily broken by removing.