Little_Nemo
Fish Crazy
He did a strip test for nitrite, but used an api kit to test ammonia levels if that helps.
Not really... the mature media should've done it's thing by morning and hopefully any damage done to the fish in the past few days is reversible.
Then it's just daily testing for a week or so, water changes when/if neccesary. The next week is a good time to start planning your tank stock long term
And then you can start to slowly stock it.
One thing I would say is though... danios are super active fish. Personally I really really wouldn't have them in that sized tank. For highly active fish there is a general 'guideline' to allow them at least 10x their body length in tank length. So you're looking around 60cm by that guide. And having kept and seen them myself I wouldn't have them in less than 2.5ft (75cm). I know it sounds silly to say 15cm makes a difference... but it really really does.
Did P@H give you a refund/store credit for the dead fish btw? As technically the fish died as a direct result of advice given instore. So they really should be giving you a refund... Not sure if the law is on your side...But if you explain then I figure it's worth a shot regardless of whether they're required to give you a refund.