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I think the secret comes to do the usual 1-2 weeks brew, then bottle with the lid on for a day or two at room temperature to build up the CO2 and make it nice and fizzy.
I found winter to be a bit harder to get that right.

If you have kids, you can use Formosan black tea (Caffeine-free), my daughter loves it, so it's plus to get those probiotics without having to pay top dollar in supplements ;).