Roots growing into septic tank

[KineticoUser]

OK, here is the final good and bad. The plumber ran his camera down the drainpipe and discovered the pipe is dry. Apparently there is no blockage. The septic tank must have settled some, as the pipe coming out of the tank has about a 1-1/2" rise per foot coming out of the tank before it slopes downward. The plumbers suggested digging a new trench around the cargo container, but this ground is too hard for a guy my age. I guess I'll just settle for the tank level being a little above the 4" output pipe. What I don't understand is how the level of the tank rises and falls. When I started this project, the level was about 1" above the pipe and now it's dropped a couple inches. You would think if it was just the slight rise coming out of the tank the water level would always stay at the same level. Do you have any thoughts on this? Under what circumstances will the water level change in a septic tank, when there is no blockage?

At least I know when I cut the pipe to put in a double cleanout in the middle and a single cleanout at seepage pit #1, along with changing the 90° elbow to a 45° elbow going into pit #1, I won't be flooded with effluent.

https://1drv.ms/f/s!AnhFeJX0YorirUgEIBl9DNKLcmh6