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There are several approaches to termite protection, none of which are mutually exclusive (so you can try all and any of them). The first is the barrier method, where you insulate your vulnerable structure with other structures that resist attack. The second is poisoning and the third is setting traps.
From what you describe, it seems that you may have already imported some infestation in the tank fill material, so it will only be possible to use barriers on future constructions. This might be difficult anyway because of the need to achieve an absolute seal between the fill and the tank liner. One approach might therefore be to sterilise the fill material before use it, e.g by roasting over a fire or by spreading out thinly in bright daylight (termites don't like daylight).
The simplest and cheapest non-persistent and non toxic termite poisons are vinegar and borax. The existing structures could be sprayed with these - probably several times before the tank is filled. The best trap is apparently wet cardboard. This is like caviar to termites, so a management strategy might be to leave pieces of wet cardboard in the vicinity of the tanks and regularly take them out and burn them before replacing with new. There is a range of of commercial termite trapping systems, many of which have to be set out around the area and buried in the ground ( a bit like slug traps).
Hope some of this is helpful.