Ooof, well this is embarrassing... I think I found my issue. I think my u-tube hasn't been locked in place. I think it sometimes moves slightly, which then leads to the bottom of the tube getting closer/further from the bottom of the in-tank overflow box. That then leads to a bit of resistance to the water flow, which leads to the hang on back reservoir's water level rising a bit or lowering a bit.
That then leads to water draining faster/slower, which then leads to the return section being slightly higher or lower, which then leads to the ATO.
Last night I fixed this by using some double sided tape to make sure it doesn't move, and since then it's been super stable.
Hopefully it continues, but that was silly.
I've also realized I have it setup slightly wrong. I have the acrylic overhang sitting inside the box versus in front of it.
So the setup should be (awful ascii art):
Code:
======= =
+ | | = + = =
+ tank | overflow | = + = =
+ | | + = =
+ ---------- + = =
+ + =====
+ +
+++++++++++++++++++++++++++++++
But I had:
Code:
========= =
+ | =| + = =
+ tank | overflow =| + = =
+ | | + = =
+ ----------- + = =
+ + =====
+ +
+++++++++++++++++++++++++++++++
Which is likely why the u-tube can move.