Mixing drum question

[Aquattro]

I used a brute can for years, no nitrates whatsoever. The can won't add NO3.
For the new tub you have, fill it with heated RO for a week or two, drain it and use. Any noticeable trace of fluoride will leach into the RO, and anything left is not worth worrying about. Lots of people use tap water with fluoride in it, worst case, fish will have healthier teeth.