Hello!
I have a 10 gallon tank with 1 small fantail goldfish and 1 snail. I thought I would buy just 1 more to keep my fish company. THe only problem is the new ones keep dying after about 2 weeks! I've tried 4 already (not at the same time) and they all died. I"ve changed 90% of the water each time one dies and I've got several test kits and the water is fine. The one fish I've had for a year and he's doing great, I don't understand why the others don't make it. They've all been really small, about an inch long including tail.
Any advice?
Thanks

it could also be that your fish are all kind of sickly. Where do you buy your fish? How big is your filter? Why are you changing 90% of the water? that screws up the biological process. the most you should be doing is 10-15% water changes, especially in a 10 gallon.

Worst thing to do is to change almost all of the water. 90% FAR TOO MUCH. Your tank never cycles in this case.
Before you introduce another fish into a tank you should keep it in quarantine for about 3 weeks, in a hospital tank

and as Kat mentioned: 10 gallons per fish, otherwise your tank is overstocked

When you test your water what exactly are the ppm (parts per mililiter) on the test results?  Any ammmonia at all showing up will make a fish go into shock and sometimes even a small amount of nitrites can stress your fish out!  It's important to test your water BEFORE and AFTER adding any new fish for the first week or two.
I agree with the rest on how important it is to quarantine the new fish.  If he has any other diseases or parasites he is carrying then you risk your "older" fish from getting those.  Fish illnesses and parasites are so contagious!

Small fish 1 inch are very sensitive.  Even an abrupt, different water chemistry from the water at the pet shop to your tank water can make a fish go into shock.  I always place my fish in a quarantine tank and float the bag for at least 10 min., then add a cup of the tank water to the bag and let it sit in the tank for another 10 mins.  Then release the bag to the new tank.. I keep the lights off too for the first 24 hours.  Then start gradually leaving the lights on for longer periods of time when I feed.  I very sorry you are losing your fish.  It might just be the petshop got a bad batch of fish too.
I hope our posts help some.

Nothing more repulsive than going into a petshop and seeing dying or dead fish in the tanks.
They could have taken the "dying" fish in the back room and put him in a different hospital tank.
People should be treated like a guest in their stores!  There are some relaly nice petshops and then there are some that don't seem to give a **** about their customers or their pets.