Did ya let the tank go through it's "bacteria load cycle" before putting them in? Goldfish are pretty resiliant . . . this may or may not be a factor . . .
As far as the bio load Furious_Male mentioned, I don't think that's a factor. Looks like about a 20 gallon tank, in which case six fish shouldn't be a problem. Mind you, seven and they all die!!

(j/k)