It is totally safe to use the spa with fc anywhere between minimum & slam level for your cya
FC/CYA Levels
It will likely not harm you to stick your arm in to get a test sample with no fc but using the spa in that condition is not recommended
(if you’re concerned, wash your arm afterwards or use a baster, syringe, or a cup with a handle to get the sample).
& get the fc up pronto!