If you have ammonia readings then it;s one of two scenarios: one, the tank isn't cycled fully OR two, you added too much of a bioload and the beneficial bacteria could not keep up.
definitely do that water change, you need to get the ammonia to 0 and keep it there.
I take the bag the item comes in and float it for about 15-20 minutes to get the temperature the same, then I put the fish or invert into a small bucket and drip to there. I use small one gallon painters buckets that I got in home depot, perfect for this kind of stuff and you can have a bunch around when you need to acclimate a bunch of things or to dip a bunch of corals.
The stress combined with the ammonia may have been the culprit. I would urge you not to add anymore livestock til you can keep the ammonia and nitrites at 0. Your nitrates arent high enough to cause any damage is the plus side.