I am sure you are sweet enough (bad me I know, had to) without the sweetner.
I have avoided sweeterness as I do not want to remind myself and get those subconscious yernings (sp??) for sugar or sweet things. black tea is new to me, so I have gone for the whole new experience of black no sugar.
has the non-tablet form got citric acid in it? It would alter the krebs cycle and knock off ketosis if so. Hence why things like Diet coke is banned.