If your other fish you bought weren't goldfish, then thats your problem. The fish more than likely weren't diseased, but it is possible. Goldfish don't exactly have stomachs, so they don't digest everything they eat properly, causing them to poop LOTS of ammonia into the water. This is deadly for other types of fish, which is probably why your new fish died. Also, if they were ALL goldfish, then its possible there may have just been too many in the bowl/tank.

Fish can be terratorial as well, so adding fish isn't always 100% safe. It can be very risky if there is a huge size difference too. Unless your new fish are super fast, they will get picked on almost always.

Hope this helps,
Take care!
JC