Is there a writeup on 2 skimmer troubleshooting
Not yet. There is a write up about skimmers in general and how they typically tie into main drains when applicable. There's possibly some other relevant info there for you.Is there a writeup on how to troubleshoot main drains
Skimmers - Further Reading

Weird and your system in post #18 seems pretty simple and straightforwardPump flow lately only reaches 70-80 gpm instead of 100 gpm, and filter pressure is only 10 psi.

