My guess is that the goldfish you recently bought were already sick at the petstore. Most of the time the store workers will remove the dead fish before the customers see them, but it has probably already infected the rest of the fish in the tank. Now if you were to buy two fish from that same tank , then sooner or later those fish would become sick and perhaps die too.

I would keep an eye on the healthy fish you brought from your mother's house to see if it shows any sign of illness if it is in the same tank as the two dead fish.

Another thing you can do is get the water tested to see if there are dangerous levels of chemicals in there, such as ammonia, nitrate, etc.  Some pet stores will test the water for you free if you bring a sample of it in to them.

So fish even goldfish can catch things like some sickness?  Learning all I can 


Jon

Okay that's interesting to know.  Thanks!

All have to keep a eye on my 3 little guys.


Jon