I am sorry, I have been for many years wrong then.
I have used this antibiotic with very sensitive fish ( discus, primarily) many times throughout the years with no noticeable effect to my bio-filter. I do have well established systems, however.
I am not saying you are wrong, because you are right, I liked it up and both Nitrosomonas and Nitrobacter are stated as gram negative.
This is mystifying to me.
This could be part of the problem:
On API's website it clearly states that Furan 2 does not damage the bio-filter.
When the company itself states this, there is bound to be some confusion.
Still not doubting the point you are making, let me be clear.
When my fish are sick, however, I change water daily regardless of ailment or medication using.
This could explain why a decrease in nitrifying bacteria is not evident to me.
I do appreciate you bringing this to my attention. This could very well make a difference on newly established bio-filters and I will now caution others that this could set-back ones bio-filter.