...Because unless you've thought it through before you set up the tank in the first place (and most people don't) option number one isn't possible. It's ideal, but even if you do it, in a few months you have to do it again. Ferts don't last forever.
And you need to understand my primary point - regular Oxmo, the one you linked, is missing certain nutrients.
But, whenever you do put root tabs in, one every few inches is fine.