auto topoff system
get a glass bottle, some airline tubing, something to hold the botte upside down and aquarium silicone. using a nail and hammer, putt a wole in the lid just big enough for the airline to fit in. place a length of airline tubing into the whole and silicone it in. silicone the other end of the tubing so the end is where you want the water level to be. set your tank up so the water level is at the end of the airline tubing. fill bottle with top off water. screw cap onto bottle. place bottle upside down in whatever your using to hold it that way and then place this above the water level of the aquarium.
This works because water does not expand easily and the only way for water to get out of the bottle is if something takes it's place (air), and the only way for that to happen is if the water level in the tank is lower than the opening of the airline tubing.