Nafish,
In replying your question, I am using a similiar diffuser connected to the outlet valve to the Ehiem 2233 canister filter. The lfs called it the external diffuser which is connected to the co2 diffuse reactor (internal diffuser) in the tank. But the external diffuser costs about RM55.

you are right that you need to follow the ADA process. I will be buying the step1 soon. According to lfs that step 1 will only be used for initial stage for the roots to take place in the substrate. Older tank will only need step 1 when you perform some replanting or rescaping. I think the rest of the nutrients are in the substrate. correct me if I am wrong.

peace