My fish died during introduction to tank?

I introduced two clownfish into a long established tank. Water tests all came out good. My nitrate was a little bit high but nothing disasterous. Salinity was the same too. I floated them for a bit and then decided to acclimate them by adding them into a bucket and slowley adding water from my tank. They did great for an hour. Swam around, explored, etc. one started laying on its side, breathing heavy and eventually died. The one that was doing the best was swimming around the tank and just stopped. Does anyone know what happened? I feel terrible and am afraid i missed a step. I just wanna make sure this dosen't happen again. 
 
Did you test the salinity, nitrate and ammonia of the water the fish came in?I lost a fish like that too. I tested the water the fish came in after, and everything was way higher than my parameters.(Ammonia 2, Nitrate 40, Salinity 1.029)So I am pretty sure my acclimation time was too short. So I always check now during temperature acclimation time, if parameters are very close to mine, then I know I don't have to acclimate too long 30-60 mins.If they are way off, I do 1-3 hours depending how far off.I would have done close to 3 hours for the fish that died on me, testing the salinity every hour.I also use the accudrip dripper and remove water from the acclimation tub every 30 mins.Before I had the dripper  I would use a turkey baster and add water every 10 mins on a timer.That has been my experience, hope it helps.Sorry for your loss, loosing fish sucks :-(
 
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