So you have an indoor pool, still air, no geological activity to speak of, is it possible to detect tides in it, say with a couple of floats on opposite sides of the pool, maybe 4 sensors, one in each corner, and a reflector of each one that can measure a few microns of height difference between each one, average the readings taken, say once every ten minutes or so and see some difference due to tidal forces of sun and/or moon?