Welcome to the forum
I will start as I often do with people by saying -> do you know about EI (Estimative Index) and have you read the pinned EI article? The article is a good place to start.
Other than that you sound like you have some planted tank knowledge already so I crack on with trying to answer your question. I guess this depends on what you mean by automated?
I have two bog standard CO2 kits at the moment, both are just a cylinder, regulator and diffuser. That's all you NEED for CO2 but as always there are plenty of things you can add. If you want complete control over the CO2 the first thing you can add is a solenoid, this'll turn off the CO2 (or reduce it greatly) during the night, when the plants don't need it. The next bit of kit you can add is a pH probe, I'm not sure how this connects up but you basically set the pH you want and the probe releases the right amount of CO2 to get the set pH. I got my bargain kits of ebay and cost me around 70quid in total each.
Have a look at
Aquatics-Online to see all the main types of CO2 kits available. For a 100G tank I would ignore the yeast type ones, you'd be better off with a pressurised kit. And the site should give you an idea of cost if you want an 'off the shelf' kit.
Right, all the above said, I personally don't think you need much more than a very basic kit -> cylinder, regulator and diffuser oh and some CO2 proof hosing to connect it all up. Quite a few people on here use solenoids, but I know some people have had trouble with their kits leaking CO2, especially when you start adding extra stuff. The big advantage is that you can turn it off at night so you don't risk overdosing the CO2 and killing your fish and it also halves the amount of CO2 you use, which may be a factor on a 100G tank.
As for the pH probes, I dont personally know of anyone that uses one (anyone out there?) and have herd that they can be tricky, and can go wrong and end up pumping tons and tons of CO2 into the tank killing all the fish, doh!
To summarise that stupidly long post, things to remember are, cost and how much you are willing to spend, how 'automated' you want to go and consequently the complexity of the system you will need. As you have probably guessed Im someone who likes to keep things simple, there are less things to go wrong if you do
Hope this helps, post some pics if you're able, we are having a bit of a nano tank revolution on here at the moment and a huge 100G planted tank would be a refreshing change
Sam
