Goldfish are a cold water fish and shouldn't be in with the Danios, Plecos or the Rainbow Shark.
What kind of Plecos do you have?
Well the cycle thing is the issue here, sorry. And the reason why it is killing the fish in the 10g and not the 50g is because the ammonia builds up faster and stronger in only 10g of water. The bigger the tank the slower the rise in ammonia.
If you can't return your fish to the LFS
you can do a fish in cycle, IF YOU ARE VERY DILIGENT.
First you must get an API FW
Master Test Kit. Then on the 10g tank you will need to test every 12 hours. If you have fish in the tank NEVER let the ammonia go above .25ppm. If the ammonia reads .5ppm you need to do a 50% PWC
, if the ammonia is at .25ppm you need a 25% PWC
. If it's at 1.0ppm then do 2 50% back to back PWC
Same goes for when you start getting nitrIte readings. Only the nitrItes are even more toxic than the ammonia. Keep doing those PWC
as often as need to keep the ammonia and the nitrites below .25ppm. Even if it doesn't kill your fish it will burn their gills and cause scaring, making breathing very difficult.
After a (what feels like an eternity)while you'll start to see nitrAtes, this is a great thing. But you can have to much of a good thing, so continue to test your water when the nitrAtes get above 20-30ppm it's time for another PWC
Or you can do what I do, after your tank has finished it's cycle and the NitrAtes are fairly stable, just do a 20% PWC
twice a week. I do my 4 tanks every Wed. and Sat morning. I have 2 55g and 2 10g tanks.
Plus you need a separate tank just for your gold fish, the 10g isn't big enough for them.
I hope this helps you and your fish.
One piece of advice, NEVER listen to the LPS
they are there to make money not give advice.
There will always be exceptions to this rule, but better safe than sorry.