Set up a laser level on the inside at the tank centre, if the tank is large enough and take level shots around the periphery... easy to do and a single person can undertake it (unless enclosed space issues). Start with a cad drawing (circle?) and a location, mark of the start and take readings every couple of paces, or measure the locations. A chunk of 2x4 x 6' as a chord distance and chalk off around the outside of the tank. Walk toward the level about half way and take another shot for each second chord distance. Did this a couple of years back for a tank. Normally measurements are not supercritical, and the method will give you a general idea of any movement. If more precise info required, then lay out a proper grid using chalk.

Dik

Unless you have initial readings immediately after construction, your new elevation readings in and/or around your tank will be have little meaning, except to serve as a basis for next years survey.