should be something like 1/2 inch tubing for anything over 20 gallons grow rez. 1/4 inch tubing would do. Need a float valve in the grow rez and it goes str8 to the top up rez. The top up rez should be put higher than the grow rez, for gravity sake. and if you can afford it I recommend getting good brass hardware or at least use teflon tape at screw joints. I also add hose clamps to any barbed fitting connections.
if the top up rez can't be raised over the grow rez, it needs to be pumped. then you would need a electrical switch on the float valve, and those cost more obviously.