Another issue, some sort of leak... What's happening?

[Bperry]

I mean it's yellow in the scale. It's in the IDEAL range. The yellow spots break up easily. But after a few days, you may see them again.

You would be better served using a FAS-DPD test for chlorine. The yellow OTO test doesn’t give you real numbers. It’s only for testing whether some amount of chlorine is in the water. The yellow spots are algae and they are there because there’s not enough chlorine in the water.