Dan,
Thank you very much for contacting us regarding our products. All of your comments and questions are valuable. We use your feedback to create the most effective line of aquarium and pond products available.
You should always have at least a half inch of air trapped in the top of the canister and this is considered normal.
However, you you begin to experience air surging, then I would be more concerned. Air surges can happen when the water level in the base drops below the bottom of the filter motor. This can be due to a bad seal or clogged filter media(pads). Your black filter pads can be rinsed and reused monthly. Replace them as needed, but they should last more than six months. The white pad is for water polishing and should be replaced monthly. It cannot be rinsed and reused or it will cause low water flow.
If your filter motor is not at least 24 inches below the top water level of your aquarium, you will have problems with air and poor water flow.
Another problem we have found is that people are filling their canister base with water and then clamping the motor on. This will cause flow problems and leaks, as you may not get a good seal between the motor and canister base, due to the media baskets moving around and not staying in place. Over filling a media basket will cause this same problem. Always fill your canister with water through gravitational siphon, as the directions show.
Any air devices run near the filter strainer or an oversized air pump used on an aquarium may cause the build up of air in the canister and cause air surges as well.
Also check the Quick Disconnect mechanism to make sure that it is seated correctly in the motor housing. An improper connection with the motor can cause air to seep in and cause air surging.
Check the small black plug on the outside of the filter motor, it is located near the power cord. If it has been pushed in too far against the motor housing, it can cause air surges. It is designed to let trapped air vent from the rotor(impeller) area. If the plug is flush with the motor, use a flat screwdriver to back the plug off just 1/32 of an inch. This will allow air to vent from the rotor chamber and may fix the problem.
If none of these situations are applicable or you wish to discuss this further, give me a call at 1-800-847-0659.
Best Regards,
Brian Bridgwater
Technical Service and Research
Aquarium Pharmaceuticals, Inc.
Mars Fishcare Group