Page 14 of the owners manual should help. Let me know how it turns out.
https://www.redseafish.com/wp-content/uploads/2016/02/2759-MAX-S-written-Manual-EN-v2019a-for-web.pdf
I also just setup a pre-owned RSM500. Ive been doing a lot of research and its pretty much cycling right now. Referring to the owners manual, and what ive experienced so far the sump level under the tank does not correlate to the water level in rear of display tank. I found that my sump level...