A belated welcome, Ceedy
As regards cycling, a water conditioner per se wouldn't help with cycling the filter (and testing the water would just give the results of the tap water with conditioner rather than after / to verify if a cycling process has occurred) - but perhaps you meant some form of bottled bacteria specifically for cycling? If so, interested to know which one you used and how the cycling process went - we're always interested to know how different products work for different people. (This article
https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/ is good for helping understand the process better.)
As for your tank, what are its dimensions in cm? That will help us work out suitable stocking, as many species of fish have minimum tank size requirements (and minimum shoal sizes, etc). As well as
https://www.thinkfish.co.uk/fish-profiles,
https://www.seriouslyfish.com/knowledge-base/ is a more comprehensive source/database of such. You'll probably notice in it that the zebra danios and golden barbs need longer-length tanks which may well account for why one might have vanished if stressed. Take a look at this
https://www.thinkfish.co.uk/article/community-creator and
http://www.aqadvisor.com/ to understand stocking possibilities for your tank a little better. If the fish are unsuitable for your tank size, you'd probably be within your rights to exchange them at the LFS for a more suitable species.
As for where the danio has ended up, I'd be almost certain that it's not been consumed by those fish - yes if you had shrimp and other bottom-feeding fish. I've had fish (one species in particular) kamikaze their way out through gaps where the cable leads run and end up on the skirting board, carpet across the room or plug extension unit, or squeeze through seemingly impossible gaps at the back of the filter between the suction cups and only retrievable once every single piece of decor has been removed altogether - dead fish somehow often end up in such places or inside the filter media.
Hope this helps.