Very much depends on several small points and your tolerance of bubbles in the tank is one of the bigger items to consider. Growout tanks where appearance may not matter, make a stream of bubbles okay, while the main show tank, you may not want to see ANY bubbles. Airstones give a pretty obvious stream of bubbles, while something more effective like a Grigg's reactor may cost more but certainly worth it as they do away with the visible bubbles.
Size of the CO2 stream is part of the question as a few bubbles per second is easier to deal than a solid stream. If you need 15-20 reactors, it can cut the cost a whole bunch if you buy parts for a reactor in groups of ten rather than single items when shopping the big box stores. How you power/filter the tanks is a big part of the question as well. Common pump system or individual filters?
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