Build a bund wall, and throw a pump in there as well. Then, just pump the liquid back into the tank to keep it full at all times. In order to deter mathematically inclined terrorists, use a variable frequency drive and make that frequency change every second. The problem becomes unsolvable.

There are other options too. You could fill the tank with tar, any really thick slurry, molten sodium, hydrogen sulfide, 2-mercaptoethanol, propane just to name a few. Drill into a tank like that and you’ll regret that decision instantly.

hear me out: klein bottle vats