AquaBotanic offers the following... pardon the lack of knowledge what all
the parts do...

"Aqua Botanic System 2 for large aquaria

75 to 500 gallons

High quality German made regulator/needle valve and featuring the Reactor
1000 external reactor, (connects to a cannister filter or sump), bubble
counter, 10 ft of tubing.

$213.20

with solenoid $273.00"
What does a solenoid do? Is it really necessary?

The solenoid allows you to put it on a timer (with your lights) so that you
can turn the CO2 off at night automatically. If you want to run reasonably
high levels of CO2 - say 25-30ppm you would need to do this because over
night the plants stop taking in CO2 and start giving it off. Therefore if
you don't stop the CO2 supply the levels can build to being to high and
dangerous for your fish. On the other hand if you run CO2 at a moderate
level - say 10-15ppm you would probably get away without this.

The solenoid is also used in conjunction with a Ph controller (which is
kinda overkill but I do like mine ;-) )This then allows you to set the CO2
levels fairly exactly since the Ph of the water will vary in direct relation
to CO2 concentration.

The System 1 they offer said it needed an additional pump, this one
doesn't.
Why?

The pump will be to operate the CO2 reactor/difusor. If the one you are
looking at has a more simple bubble ramp or spiral then it doesn't need a
pump - but its also not as effective and would not allow you to get to over
20ppm in my experience.

If you do a search of the web/google there are lots of ideas for DIY CO2
reactors - everything from converted gravel cleaners through upside down
internal filters through people who bubble it into the intake of an external
cannister filter. This is what I do although I have a pond cannister filter
in line with my Eheims as a pre-filter. The water collects from two inlets
on the tank, down into the pond filter which acts as a mechanical filter and
then out into two Eheim cannisters which are stuffed full of Bio media. I've
only just put this together and its working very well - the plan is to clean
the pond filter every couple of weeks in tap water to actually prevent any
bio filter growing in it - that way all the bio work will be done in the
Eheims - which should almost never need cleaning.

Because the pond filter is top entry - the water goes in and down the
outside and then comes back up a tube in the middle (past a UV light) so it
also happens to make a very efficient CO2 reactor! My Eheims never "burb"
CO2.

HTH

I.