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Just a comment - you have presumed the damage is due to termites, but you may want to consider other options. Are there any other signs of termites such as actual sightings or tunnels in the sand layer?
Other causes may be damage during construction or transport. Are people standing in the tank during construction or cleaning? Is the sediment in the tank gritty? Was the protective sand layer soft like sand for mortar or harder like sand for concrete?