I'm sure this question pops up all the time and it probably gets quite irritating answering it all the time, but here it is.
so what is the difference if any?
should I have a CO2 thingy even for a 5 gallon aquarium that I have?
Best is one thing, what you need may be something else
While a reactor is best, that is most efficient, what one needs in a specific tank may be a diffuser.
No, I take that back, some reactors are not more efficient than diffusers. So, it is hard to say.
Where space and money is tight, I'd suggest diffuser. In a big, big tank, where getting the CO2 to all plants in the tank is important, a power reactor is the way to go, and a power reactor can be external.
A diffuser breaks down the CO2 bubble into many tiny bubbles. Since CO2 is not readily dissolved in water, by breaking down the CO2 bubble, it will enhance the dissolve rate of the CO2.
As for reactor, what it does is spinning water from filter output with the CO2. By creating a cyclone effect in the reactor, it prolong the duration of CO2 in the water and with that aggregation in the reactor, the dissolve rate is highly maximised.
Right now, I am using a diffusor for my 10 gal and a external reactor for my 90 gal.
Kevin, I am willing to bet if you hooked up a good power reactor, you could turn the flow rate down lower and maintain the same pH c02 level, or perhaps even lower ph (more c02) that the level you are getting now using the diffusor. How often do you have to clean that thing? How long does it take to get covered in algae? I fought with those things for over a year and swore I would never even so much as look at another diffusor for the rest of my life! You want cheap? Buy a little power head with a sponge filter attachment and connect the C02 line to the top of the power head. It will spray out a fine mist of bubbles. It still wastes a lot of gas compared to a reactor, but it is still 100 times better than the Eheim diffusor...
Power reactors kick butt. The best money you will ever spend.
Art, Kevin is not the one with a five gallon tank. I agree, for a tank that small a bell type diffusor may be the best alternative. Any reactor would probably look to big and be too much of an eye sore.