The description of that type of light amplification between two mirrors is accurate, I get what you are referring to. That is as you know called "photon pumping." The mirrors collimate the beam, thus raising its effective energy a lot. But raising photons to a higher energy state is not what's happening between two synchronized pumps. In the case of the pumps, yes you can find a natural frequency between them that increases the waves height, and places a bit more strain on the tank perhaps, but not very much as again, it's not focused. But in no case I can think of other than creating larger more focused and higher velocity shock waves, or dumping a lot of water into the tank all at once (major PSI changes) could you damage the tank. If you built the wave high enough, it would overflow the tank sides. Think about the old example of using a electric fan directed on a sailboats sail. Only works if the fan is not in the sail boat. Interesting exercise though.
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