Although
Goldfish often change colour, usually this is the result of a loss of dark pigmentation. For a
Goldfish to gain darker spots is very rare. In other words,
Goldfish commonly get lighter with age but darkening with age is nearly unheard of.
I am more convinced that the dark spots you are seeing (espescially if they have come up over a period of days or weeks) are ammonia burns. Do you have any idea of how many PPM your ammonia is testing at currently?
It's very (WAY too) common for people to 'overstock' their tanks with
Goldfish. Basically if you are not running at least 10g per
Goldfish and overfiltering your tank you are in for a lot of trouble. To determine if overstocking is causing an ammonia spike please answer these questions:
-How big is the tank?
-Are your
Goldfish 'fancy' or 'common'?
-What is the make and model of your filter?