I think I'd go for a float sensor, still - but not a "swinging arm" one; a 500 L tank is not small, so I don't think a swinging arm sensor would work. What would work though is to use a multi-turn potentiometer, possibly geared in such a way that your "float", resting on the surface of the water, is slightly heavier than a counterbalance weight. The potentiometer, gearbox, and counterbalance weight all are located outside of the tank. The float is connected via a cable wrapped around a knurled shaft (or between a couple of rubber pinch rollers), which is attached to the potentiometer via gearing (so that it takes multiple turns of the gearing to turn the potentiometer slightly); the counterbalance weight is attached to the other end of the cable.
As the water rises (as the tank is filled), the shaft rotates one way, as it falls (tank being emptied) it rotates the other way. I'm actually pretty certain there are more than a few commercial instances of such devices out there.
You might also look here for some other ideas - which might be possible to make in a DIY manner: