This very topic is discussed on a daily basis. Instead of just starting a new thread, it would be very much to your advantage to use the site search function. I am sure you will find more info than you want on this very subject.
Keeping everything said above in mind, let me mention one method that has been working well for me.
I just pass the CO2 outlet with an airstone into a piece of foam which has a downward flow of water thru it! That's it. Almost 100% dissolution. Currently I am using this method in the wonderful subcurrent internal filter and it works great. The method involved is not too different from inline reactors used with canister filters. The CO2 bubbles try to rise up. The water tends to force them down. They get trapped within the foam in flowing water. And voila! Simple, cheap and effective and no additional junk in or out of the tank.
I have used multiple methods. One is kinda like the one above only i used a powerhead the end of a gravel vac and put the airstone in that and pointed it downwards. I have also used ceramic diffusers, vortex reactors, and inline canister filter reactors. On my large tanks i use the inlines only because they can be hidden under my tanks, and on my smal tank i used a ceramic diffuser. It's all a matter of preference as stated before. they all work it's mostly just aesthetic value.
Thanks for all the info. I looked at the other threads on this topic, as suggested above, and saw that some people were swearing by Rex Grigg's in line co2 reactor. Then I checked another thread where someone wrote that this reactor " cuts your flow in half, if you're lucky." Anyone have any experiences with this reactor?
Anything you put inline will restrict your flow to an extent. I personally don't use them because I like to maximize the flow from my canister. I've diffused through my canister filter, through a powerhead, with a limewood airstone and have found my best success using a Hagen Elite mini filter underneath a Koralia powerhead. There are probably an endless number of methods that people are successfully using, you just need to find the one that works for you.