… about my weird cycling experience, with ammonia spike apparently non-existent and nitrites rising. He didn’t know either, but pooh-poohed my suggestion that maybe the “bioactive” dechlorinator had something to do with it after all.

My new theory is that it might have something to do with the bogwood that was soaking in my tank for several days before the fish arrived. That seems to be the only possible remaining source of bacteria, either bringing them to the tank itself or (perhaps more likely) producing some ammonia which attracted them and started the cycling process before I even got the fish and began measuring levels in earnest.

Whatever, the nitrite levels continue at 0.3mg/1 – controlled partly by water changes but mainly, I fondly imagine, by the growth of nitrite eating bacteria – so the cycling process seems to be progressing. More importantly, from my point of view as fishmummy, the cherry barbs are out and about much more now and seem to be recovering well from the nitrite stress. All is well.