A fluid level controller which replaces fluid lost from a container thus maintaining the fluid level in the container. It is comprised of the following major pieces: an airtight reservoir (10) with cap (14), fill stem (12), feed lube (16), and tube attachment fittings (18a and 18b); several lengths of fluid conduit, (22a,b) and a u-bend (26) which form a siphon such that one end reaches into the bottom of reservoir (10) and the other reaches into the container below the desired fluid level; a length of conduit, (22c) which forms an air return line allowing air into reservoir (10); a stand (8) atop which sits the reservoir; a tubing clip (24); and a shut-off valve (20). The u-bend (26) allows the controller to preserve the initial prime of the system so that it only needs to be primed once. The shut-off valve (20) allows easy refilling of the controller once the fluid reservoir empties. These two features make the controller extremely easy to use. When set up correctly this system uses air pressure to replace fluid lost from a container. Fluid is replaced at any rate up to a specified upper limit, in effect controlling the fluid level in the container. In operation the controller has no moving parts and uses no electricity, thus the mechanism has an extremely low risk of failure. This device has been shown to be an effective, safe, reliable, and efficient means for relieving some of the maintenance associated with owning an aquarium.