Fish Help! What Did We Do Wrong?

Several things could have happened. If the temp in the tank and the new water was too different they can go into shock. Was the 20gal tank new? People often make the mistake of cleaning tanks and supplies w/ soap a big no-no. how long had he been in the new tank? if it was, say a week plus, it may have just been his time and the tank change had nothing to do with it. Koi usually live a long time but you never know. Where is the new tank? If not on the same bookshelf, again, temp may be the culprit. Fish are highly sensitive to temperature changes. It may be too late now, but are you sure it was dead? They will go "dormant", and appear to be dead, especially if the water is cold, esp Koi, to conserve body warm etc. recently my son thought one of his fish died but when I looked closely it was fine. If it was floating on its side completely, yeah it was dead, kinda sideways, may not have been. Have you done a water test? All carps/goldfish etc are notorious for producing a lot of waste, and ammonia, when you shifted the water and by extention the gravel in the tank you may have stirred up a lot of waste, too much for Shimmers to handle. The filter: was the carbon filter rinsed properly before use? They tend to have a lot of dust in them, usually not a big deal but you never know. Shimmers may have been too stressed from the move, everything may have been fine, but the move itself may have been too much. They sell a stress relief to add to tanks and bags. Most petstores actually put some in the bags when you buy them. I would do a water test before buying another fish just to make sure. Sorry and good luck

When I did this...I would always put my fish in a ziplock and float them on top of the new water for along while...but first waiting until the new water was room temperature....and making sure to use enough of the amonia drops. I never had a problem...the lived many years, R.I.P.....Cuff and Link. :(

We've always had fish tanks and NEVER changed the water on any of them. We strictly would fill the tank after it started to evaporate and squirt some of the water purifier and done. Although my mom would drain about 1/2 the water out of the big 60+ gallon tanks, leave 1/2 the "dirty" water in it and fill it and put in the purifier. Our fish lived forever! I even have Beta's that have lived for YEARS. The other thing is that my mom always said "a hungry fish is a happy fish". She fed them maybe once every 1.5 to 2 weeks or so. Well, that's what we do and don't have many casualties but I know others do differently. I hope you figure it out! Good luck!!!

It could have been sick:
http://aquariumlore.blogspot.com/2006/04/koi.html

Koi also do best in larger groups of fellow koi fish. They can be high maintenance fish to take care of. It may have been best to not transfer the old water from the old tank. Perhaps the temperature or the chemical levels weren't correct.

http://www.abcponds.com/koi-ponds/koi-care-tips.htm

I had fish growing up but have not had any in about 5 years. When we changed the water we would allow the water to sit without anything for about 24 hours. It often meant that our fish stayed in a bucket for that time. If you can use gallons of Spring water instead of tap water that was always the best but I can see where it isn't feasible with a 20 gallon tank. Then we would place the fish in a plastic bag - like they do in pet stores - and float him on the surface of the water for about 5 hours before making the transfer. Sorry about the loss. My dad has a pink gourami that he has had since I was in middle school. This fish is immortal I swear!

We got a 20 gal fish tank for "fun" because they "are so easy!"....NOT...I was shocked by all the stuff you are supposed to do to these tanks. We (like you) had "fair" gold fish we won who lived FOREVER in a small little dish of somekind...never conditioned the water etc...but then years later as an adult I buy this big tank and all hell breaks loose! We have now had the tank for about 2 years...and the first year we lost SOOO many fish...poor things! Its finally stable...now that I am tired of them and ready to get rid of the tanks LOL....Anyway, sorry for your fish, I feel you pain!