Fish problem??

Ok, well we have a 50 gallon tank in my basement that houses about 6 pretty large gold fish. They have been in there for years, and are happy and healthy (None have died in about 2 years) But I have a ten gallon tank (what a goldfish needs...?) that has a momma guppy, and her 4 babies. Well, I decided to get a goldfish to spice up the tank. However I knew that the goldies would eat the guppys, so I put a "breeder" tank (a smaller tank that fits in the larger one) in so they would be safe. At the petstore, I decided to get two new gold fish, but last night, 2 days later, one died! What? So I moved the remaining fish to the bigger fishtank. Well, it gets worse... Today at like 4, I returned the dead one, and got a new fishy. Well, I came home today to find the new goldfish lying sideways on the bottom of the tank! I imediatly moved him to a smaller tank filled with bottled water, and after rigorous help from me, he is steadily getting better. But the guppies, in the first tank are fine?