Horizontal pipe is a problem because of the amount of air it often will contain. "Self-priming" pumps have some capability to deal with air, but not lots. And, what capability they do have is usually lost at low speed. So, it's necassary to start on high, run long enough (15 minutes?) to make sure priming has occurred, and THEN switch to low. Putting swing (not: spring!) check valve inline BEFORE the pump can help, but be CAREFUL to orient the valve properly, make sure it's testable and serviceable (debris can get caught in them), and that it's a good valve.

OTO is the only almost 'bomb-proof' chlorine indicator: if says you have none; you have none, and if your chlorine is VERY high, it will still indicate. It's also cheap.

The DPD-FAS system in the K2006 is MUCH more accurate, but not quite a bomb-proof. What many experienced folks here do, is use OTO for quick daily testing, and DPD-FAS for periodic complete tests.