I wouldn't really worry about the relay for a auto top off. The relay is more specific to the volt surges when the pump kicks on.
If you use this float
i-float Float Switch
With this pump
Tom Aquatics Aqua Lifter—Dosing Pump (Saltwater Aquarium Supplies > Overflow Box > Accessories )
You will be ok. Just wire the float into a normal white or brown extension cord and it will work like a dream. This is the setup i use on my top-off and it works great. Been using it this way for about 7 months now.
The failure is from the float not being able to handle the spike. The floats you get pre made from marine depot or someone else can handle the volt pike. The
DIY floats can't handle it so you need the relay. But with the aqua lift pump is doesn't generate that much of a spike so it won't harm the switch. When the switch has a large spike it will heat the connections together inside almost welding them together and the float and won't let the pump turn off or on.
