Question
I brought in 6, 4-inch goldfish and 10 babies from the outdoor pond for the winter.  They have been fine for over three months but now they are dying suddenly.  I'm down to 3 babies and 2 larger ones.  I have done nothing different since I brought them in, they have a filter pump/waterfall, and get fesh water every week.  were healthy eaters but will not eat now.  This started two days ago and they are nearly all gone, any help in this matter would be greatly appreciated.

Answer
Have you tested the pH, hardness, ammonia, and nitrite levels in the water?  What were they?  My first thought is that the ammonia levels may have gone up as the fish have gotten larger and been in there for a longer period of time.  How big is the indoor aquarium (or is it an indoor pond)?  How much water do you change each week (% of the tank/pond)?  Do you add dechlorinator to the water?  What about salt?  Salt can help with some problems but not having it certainly shouldn't have been a problem.  Do the fish show any symptoms?  

I hope you or I can figure out the problem and save the remaining fish!  

Good luck!