yes use a conditioner to make the tap water safe, but dont use it more than once.
it sounds like you're using it to keep ammonia levels down.
in which case the reason your ammonia levels arent going down on their own is that you arent let any of the ammonia eating bacteria grow, because if it doesnt have any ammonia, what is it going to eat?
you NEED that ammonia eating bacteria.
therefore, since you dont have any ammonia eating bacteria, everytime your fish produce any waste (which is probably pretty often) the ammonia levels in your tank spike up (poisoning your fish) and then you get rid of the ammonia before it has a chance to start some ammonia eating bacteria. therefore you're probably going to spend lots of money trying to get rid of ammonia every day of your tanks life.
it's unfortunate that your fish have nowhere else to go for the time being, because the extreme fluctuation in ammonia levels, they will either die, or their life span will be significantly decreased.
i suggest you use less than the reccomended doseage of amquel if you insist on using it, that way it wont completely get rid of all the ammonia, and SOME benificial bacteria may be able to colonize.. then gradually add less and less amquel until your ammonia levels are NATURALLY zero. but i'm guessing that could take months.
good luck.
*EDIT*
HOWEVER since it's a 20 gal tank, and zebra danios are fairly small, i think they will be alright (they wont die i dont think) as long as there are only a few of them.