aquarium top off

Not everybody has the skills or budget to install a auto top off system for their coral tank, or sometimes the tank is so small it's not really worth setting one up unless you see a ton of evaporation happening. Aside from just manually adding in fresh water to your tank do you have any creative...

I'm setting up a tank for a client and I bought a canister filter for it. My only question would be when the tank is fully set up how do you top off the tank properly without endangering the tank inhabitants?