Advice to *anyone* who uses a maintenance service. Check references. They are not all as good as another.
Test the water yourself, regularly. You should receive a written report each time with parameters and observations as well as a description of work performed etc. If you don't, find another service - don't even waste time asking why they didn't. Do test to make sure the results are consistent. Trust - but verify.
Speaking from direct experience, I can tell you that a LOT of so-called services don't ever bother to test - and I even wonder if some have even changed water :-/ It's an unregulated industry so there are not set standards.
Not naming names or singling anyone out - just a suggestion. Over the years I've seen and fixed some real messes where somebody paid good money for 'maintenance'.
Jenn