The float drains a tank when it reaches the upper limit because water flows in via the long opening causing it to sink, the reservoir is empties. There are 2 places for weights which can be sealed with aquarium silicon. Of course there are advantages and disadvantages to this system. 100mm square 30mm deep with 25mm spout.

I don't know who designed the first of these, but see how it works here. https://www.youtube.com/watch?v=OyEWJNbKbVo&t=56s&ab_channel=MyAquaponics