There is another thread about measuring the level in a petrol tank (sorry there is a bit of a cross post). Looking at that I came across the fact that there are piezoresistive sensors available. One of these could simply be bolted into the bottom of the tank. It sounds as if they may be sensitive to temperature but I guess in a fishy environment that will be controlled or at least measured and so could be compensated for if need be.
I get the idea with the oil and air now, neat, and you could always use codliver oil.