Welcome to the hobby and as for ATOs you have quite a few options in all sorts of prices. The Tunze mentioned is great, I run a different Tunze ATO on one of my tanks and love it. On my 3.5gal pico, I use a float switch that is controlled by my Apex, same in my 20L. I have also done the Aquahub DIY kit that Bulk Reef Supply has and had good luck with that. I have also heard good things about the Autoaqua Smart ATO systems. There are a lot of good ATO systems out there so shop around and see what fits your budget and size requirements. Don't forget to take head pressure into account when looking at the pump the ATO is using in relation to where your top off bin is located. If you have a specific model in mind and have some questions, then ask away.