the DOC is just the skimmer, the reef pack is a skimmer and an internal filter. Go with the 9002 Nano DOC Skimmer or the Nano Reef Pack if you want a tunze skimmer
btw ATO stands for auto top off. When water evaporates, it is mainly water that evaporates. Only very very very very (x100) small amounts of salt and minerals is evaporated. So when you top off tank water you should always use pure water since that is what has evaporated. An auto top off is a system that uses either optical sensors or float valves. Once the water level drops the sensor/float valve turns a pump on. That pump has a tube going into your tank and it pumps pure water in.
http
/smg.photobucket.com/albums/v313/mus...nt=CIMG1035.flv
that is a video of my ATO, it shows you the controller where all the electrical stuff happens in (the panel with those lights), it shows you the bucket, inside the bucket is a pump with a black tube coming out. In the tank you should see two things, a little black arrow dot thingy, this is an optical sensor, and there is a float valve. The optical sensor is the main controller since those are normally more accurate, the float valve is the back up.
Of course you dont need one of these, you could manually top off daily. Im just lazy and i want a stable system as well as having my skimmer work to its full potential.
Its a tunze ATO, cost me 178 dollars,
one of the best ATO systems for sale.