You have the outlet almost completely blocked on the skimmer. Open up the outlet and the skimmers water line will decrease. Then give it a few hours to build up a foam head. The water line mark should be a line on the skimmer itself.
Check how old your test kits are. Old test kits or test kits that have passed their prime will give inaccurate results consistently. I would double check the nitrate results by taking a water sample to the fish store and getting them to test it. Or, find a friend that is also in the hobby who is willing to test your water for you.
What is an automatic overfill?
I know of automatic neck cleaners, automatic fish feeders, automation through controllers.. do you mean Auto Top Off Units (ATO?) Yeah,... I know a little bit. Measure your space.... buy a container (glass, acrylic, food grade plastic) for your reservoir. Pick an ATO off a website - like a JBJ unit - or a Tunze Osmolator. Install according to the directions and your set! Or, there is the semi-poor mans method - which is having it gravity feed and installing a float valve in the sumps return pump chamber. And then there's the Dirt Cheap Dirty method - which is using a container and some airline tubing with a couple of knots in it and you try to match the evaporation rate as closely as possible.
