I sent a message to you Adam.
I decided to add the edited version of the message here to since it does have some info in it that I have no previously posted.
I do have a micron filtration pad, I think its 10 micron pad. Im going to remove that tonight to see if water flow and the air bubble problem goes away. Hagen says that micron pads may cause the air problems if they clog up.
My question is, understanding that a clogging micron pad could create excessive vacuum between it and the impeller, where is the air coming from? The only area I can think of is the o-ring at the impeller and motor. Some day, not this month but I may take some aqaurium safe silicone and apply it to the o-ring and install the impeller and motor to the filter housing, install a micron pad and see if the air still gets in.
Hagen told me that if its an o-ring problem, then water would leak from there too. This would be true if the oring was bad. Negitive Pressure (Vacuum) and Positive Pressure (well.. Pressure) can have different effects on the sealing properties of a gasket such as the o-ring. Pressure could seal it and to much vacuum could potentially reduce the seal, but since there is so much vacuum, no watter escapes.
Another thought on this same issue is that I THINK the o-ring for the impeller and the motor is on the HIGH PRESSURE side (back of the impeller) and not the NEGITIVE PRESSURE side (infront of the impeller) so that particular o - ring may not have any issue at all.
Now for the flow issue, I have no idea why my filter flowed so horrible at one point. Must have been trapped air some where, im guessing at the top of the filter unit which in theory, when the filter is off, it should all escape through the output.
Considering the water has to change directions atleast 3 times which is VERY bad for performance, any marginal clogging could reduce the output drastically and effect the output more then say on the Rena. Not saying the rena will last longer, but infact saying if the water only had to change direction ONCE like on most canister filter, especially the Rena Xp Series, then the effect of marginal clogging would not be as severe.
I only know
very little about fluid dynamics but I do now that change in flow does have an effect on pressures within the unit. Now how much of an effect its really has, I really do not know.
So with that said, im going to remove my micron pad and see what happends. Micron pads should be changed or thoroughly cleaned every 3-5 days according to Hagen which seems about right. I always try to get months which doesn't help.

. Im bad I know.
Justin -