If you can not keep alk low for pellet, how can you keep low with vinegar? And with biopellet there are many reports that works safe in higher alk.
Organic carbon dosing whether soluble organics like vodka ( ethanol) and/or vinegar(acetic acid ) or more complex carbohydrates like bio plastics( pellets) may have a small effect on alkalinity as the heterotrophic bacteria use some bicarbonate . Some of which may come back when acetate ultimately formed is metabolized .
All in all there is little if any effect on alkalinity at least none that is detectable ,IME, with any of them whne dosed at equal levels.
The heterotrophic bacteria compete with the the ammonia oxidizing bacteria for ammonia and actually take up less bicarbonate than the ammonia oxidizers do and often outcompete them . So, in that regard there may be a less alkalinity used by bacterial activity overall in a specific aquarium with organic carbon dosing . However, the nitrate formed by ammonia oxidation puts back some of the alkalinty when it is reduced via anaerobic bacterial activity .
As for cyano, it sometimes occurs with any of the organic carbon sources including vinegar,vodka and pellets. I suspect it's more of a start up issue related changes in competition for less nutrients that gives cyano a temporary advantage until bacteria establish themselves fully. After the first month or so , I haven't seen any notable cyano in my system and have been dosing a combination of vodka and vinegar for about 6 years.
I prefer soluble organics( vodka and vinegar) because:
it's much easier to get a precise and constant organic carbon dose than it is with pellets;
more complex carbohydrates like pellets, sugars, rice , etc have a much longer degradation pathways including breakdown to monomers some of which may be harmful,IMO.
