brown sludge

Question
We upgraded from a 10 gallon to a 30 gallon aquarium for
our quickly growing goldfish.  Everything was set up according to specifications and our fish seem to be happy.  I feed them 2x a day, not much food and I know they are "dirty" fish.  We have a brown sludge consuming our tank, the glass, rocks, plants and this porous rock we have.  The lady at the store which we purchased the tank said it was either 2 things:  overfeeding or not enough light.  The light is on for a good period of time and I feed them 2x, but little amounts.  We have tried to clean the tank and other items, but it came right back.  Any suggestions on what our problem may be?  Thanks for your time and help.

Jen

Answer
I don't agree with the lady you spoke with.  I think too much light causes algae in the tank. You don't mention what kind of filter or air you have going.  You also don't mention how many goldfish you have.   Did you have this problem in the 10 gallon?