How do you describe the work stages?
An organized company describes its work easily: inspection, draining, removal and scrubbing, disinfection, rinsing, final check and handover. If the answer is generic like "we clean the whole tank", ask for detail; someone who knows the job explains it with confidence.
Notice whether the company mentions actual removal of sediment or only a rinse, whether disinfection is a separate step, and whether there is a final inspection. Those three define quality.
If possible, ask what they do differently for a concrete tank compared with a plastic one; the answer shows flexibility. Write the steps in a note and compare against them on the day, and if a step is shortened you can object clearly.
