My additional piece of advice: get a pack of denture cleaning tablets. These are especially good if you use your bottle for anything other than water (squash, coffee etc) or if you’ve got a built in straw with awkward curvy bits.
You put that tablet in the bottle, add hot water, let it fizz and soak for a bit and hey presto, any stains or discolouration or weird little crevices are suddenly removed of their hidden nasty bits.
My niece kept saying her water bottle tasted weird, and she washed it and washed it, and then me and my mum were like GIVE IT HERE and we put a denture tablet in it and added the straw to it and it started fizzing up the straw and all this black gunk started coming out the weird curvy bits of the fitted straw like a Coke-mentos experiment.
It’ll taste slightly minty unless you rinse rinse rinse rinse rinse rinse but that’s not a terrible thing, and let’s face it, denture tablets are for cleaning dentures so, you know, designed to clean things that go in mouths.
Anyway: wash your water bottles! Wash your flasks! More often than you think you need to!! Keep denture tablets in the cupboard!!