Bill D,
I'm so curious as to how some people don't have to pump for 15-20 years!

My filter is not between tank 1 and 2 but between tank 2 and the pipe that goes to the sand/drain field. It is there so if the solids in tank 1 overflow and then fill tank 2 that they don't go and plug up the drain field. So far no issues and only liquid in tank #2. The pumping people around here quote about $350 to pump both tanks and they say that I can ask them not to pump tank 2 but their usual policy is since they are there, pump both.

My drain field is oversized on purpose since we had the room and hopefully it will last. There isn't a great option 2 on the property for a back up drainfield. The specs for our house called for a 2000 gallon main tank and a 1500 gallon secondary tank, we went with 2 2000 gallon tanks.

I'm eager to hear if others felt adding bacteria, or stirring the contents to add air or something similar was the key to better performance and less pumping opportunities.