As per usual...the devil is in the details. The sensor needs to be small, lightweight, battery powered, and most importantly, the tank must operate on its own (the tank will indicate that it is empty by illuminating a led on the exterior of the tank. The liquid may not always be water, and may not always be clear, so optics will not work.
The only reason that I went with capacitance is because of one thing that I didnt mention...it has to be kept small, and self-contained. I cant think of a way to weigh it, and keep it small, and self-contained. I tried a test with a pre-made, self contained capacitance proximity sensor, (THE ONES THAT LOOK LIKE A HUGE PLASTIC BOLT) on the bottom of the tank, and it worked....except for the sloshing. I cannot use a pre-made unit, as all the ones that I have found either require voltages that are so high, that batteries would be prohibitive, or the sensors are physically too big. I would also just prefer to make something from scratch, to try to learn something.
I found a sensor at:
https://www.electro-tech-online.com/custompdfs/2006/06/capgage-1.pdf
and was wondering if it would work by wrapping 2 pieces of foil around the exterior of the tank, leaving 2 insulating small gaps down either side. The 2 pieces of foil would then become my capacitance plates. Its really small, SIMPLE, and can be run off of a 9volt battery.
So now we must address proper filtering if Y'all think that a capacitance sensor will work. Unfortunatly Im rather old-school, and know nothig of stamps, pics, and microcontrollers, so is there a way to do this a good old-fashoned analogue way?
I REALLY appreciate the time that all of you have taken to supply feedback, and help that you have given me so far.