R.i.p. Squirt

So I checked my water peramiters this morning and noticed my nitrates were a little elevated, 40ppm (seemed darker than 20 but hard to tell on API kit), amonia and nitrite were both 0, so I did a 50% water change a few days earlier than I usually do, which is the usual amount I change weekly in my 20gallon.

When I was doing the water change I didn't see my smallest panda Cory hiding behind a rock right beneath where I was poo ring my bucket into the tank, I felt bad when I saw him get swept up rather quickly but he seemed to just be a little shaken as he swam away to hide beneath a plant in the corner of he tank. About an hour later though I went to feed my platy fry and noticed he had his tail buried in the sand and his gills weren't moving so I just wiggled the top of he plant he was near and as the other cories all swam away he just kind of floated onto his side he had passed.

Could me scaring him with the water change have stressed him enough to kill him? (Not sure if he was a he or she as he was by far the smallest Cory in the tank, only about 3/4") he's the one on he right in the pic. I've only had him about 5 days but he's always been the most active Cory in he tank and never showed any signs of stress before this. I will be able to get a replacement from the shop I bought him from but it saddens me to lose any fish, especially one as outgoing as Squirt.

Sorry for your loss  Squirt
 
It is hard to tell what may have killed him as I have lost Pandas from an unknown cause.  I know they are more sensitive than some of the other Cories to pH etc.  Obviously whenever we add new water there is a risk due to subtle changes and if a fish is bombarded with new water who knows.  Do you know the parameters of the new water vs the old?  Was it at the same temperature?  What additives did you add prior to adding it to the tank?
 
TIP: I tend to use a jug to re-fill a tank, or in the case of larger tanks I pump the water back in, this way there is less movement within the tank and the fish can adapt slowly to the changes.  Also, it protects my back from injury
 
In the case of Squirt you also have not had him that long to have said if it was the WC that killed him.  Do not be too hard on yourself, accidents happen and in this case you may not have been the cause.
 
Not sure if there is a Panda Cory guru out there who may be able to advise further.

RIP Squirt! Sorry for your loss, Fresh!
It may have been the shock of the water. Might be worth syphoning the water back into the tank rather than bucketing it in.

little squirt!! 

Ty everyone, I'm not sure what the exact stats on my tap are, I do temperature match with a floating thermometer with every WC though, and I use 10ml of Stress Coat+ added to the bucket while filling and allow it to sit about 10 minutes. I think I am going to try the jug technique or maybe just buy one of those water changers that connect to the faucet. How do you safely dechlorinate the water when using one of those? Would I just add directly to my tank before filling? Also do you think a 50% change could be too much? Maybe 20%-30% would be easier on the fish.

RIP Squirt.
Don't stress too much, his passing may not have been a result of being caught in the new water stream. It could have been any number of reasons. 

May he forever swim in the big lake in the sky! R.I.P