Is the tank a biorb? In that case, you shouldn't have too much problem with changing the sponge as those big stones on the bottom of a biorb are actually the biological media. But I agree that it is much better to just wash the sponge, that way you don't lose any of your bacteria. You still need to keep an eye on your ammonia and nitrite readings because although the stones hold a lot of bacteria there would have been some on the sponge as well.
What exactly did the shop test for? You need to know the ammonia and nitrite levels as both of those are poisonous and should never be allowed to get over 0.25. Nitrate is less important as it is only poisonous at high levels. The pH isn't too important either so long is it's not too high or too low. If they didn't test for ammonia it could well be high and causing the problems with your fish.
And as MrsM said, the biorb is much too small for goldfish. They will grow much bigger than you think and as they create a lot of waste, they need oversized filters.
What exactly did the shop test for? You need to know the ammonia and nitrite levels as both of those are poisonous and should never be allowed to get over 0.25. Nitrate is less important as it is only poisonous at high levels. The pH isn't too important either so long is it's not too high or too low. If they didn't test for ammonia it could well be high and causing the problems with your fish.
And as MrsM said, the biorb is much too small for goldfish. They will grow much bigger than you think and as they create a lot of waste, they need oversized filters.
