Auto Top Units have 2 types. One that goes directly from the RO/DI unit directly to your sump. This is what you asked for. For this type, you use what is called a solenoid to control the water.
Here is a link of one that I use. It is really good and simple to use:
http://www.bulkreefsupply.com/store/level-loc-auto-top-off-unit-for-ro-systems.html
To install it is really simple. You insert the output of the RO/DI line directly into this bracket. Then, adjust the bracket to the correct height. What I do is run it on a timer. The unit uses an electronic and manual float to add protection.
The problem with this type of setup, is for some reason, if it fails, you can flood your tank as well as your home if something blows up.
I do use this setup and this system goes for around $99. Truly simple to use and has worked great for me.
The 2nd type of setup, I have on a Solana Nano tank. This is where I store a 5 gallon bucket of RO/DI water. What happens is you install a float into your sump that is controlled by the ATO which has a turn on and turn off for a pump located in the reservoir.
2 common manufacturers are ATOTopOff.com where this is a nice setup but lacks good documentation. The other popular vendor is JBJ Auto Top Off that is sold by a number of vendors. The JBJ has a lot of cool bells and whistles. Both are considered good.
I think the JBJ can go for around $80 to $100 and the setup for AutoTopOff.com has separate prices for each part of the setup.
The advantage of this setup is that you can only flood your system in terms of failure on the size of the reservoir.
These 2 setups are the basic setups. You can do a combination of the two. For instance, I use a timer, so mine turns on a couple of times per day.
I used the Level-Loc for about 2 years and it is very easy to use. The AutoTopOff.com setup is very good. I just wish documentation was better.
