I have a 2' tank and wondering the best way (proper) to distribute CO2 inside this kinda tank. Would it be bettter to use external reactor or should I go for diffuser ?

I am really confused. Please help.

I am not an expert in this, and am still learning and experimenting, but the "ADA-knockoff" ceramic disc diffusers sold on eBay by AquaticMagic have worked pretty well in my 20-long (30in).

Also, putting the diffusor right below or next to the filter intake seems to help distribute the bubbles. I have not done a scientific test of this and it sometimes causes my Rena Filstar XP1 to "burp" occasionally, but the spray bar seems to throw the bubbles all over.

The "ladder" type did not do anything for the tank, just seemed to waste CO2...

I have been trying to make one of the inline reactors out of PVC to experiment with, but my construction skills are still nascent.

I used an Azoo or a decent no name brand glass diffuser etc.
then added a 250 GPH canister filter and blast the mist all over in a circular pattern around the tank.

Works well.

Regards,
Tom Barr

Check this out: Another method for CO2 diffusion: needle wheel powerheads