Join Date: Jul 2010
Location: Outside Philadelphia, PA
I see two possible issues.
First, goldfish are a cold water fish, and like temps a bit lower than most tropicals.
Second, goldfish because they are cold water fish, have a heavy slime coat, which they "shed". Ever notice that goldfish tanks are seldom crystal clear, even when it's fancy types, not crowded in larger tanks?
My choice would be to test your water and add ferts as needed, possibly at a lower level, rather than the full recommended rate. Remember that your not going to get very complete fertilization from fish waste products alone.
My second choice would be to just add more regular fish to the tank. Remember, goldfish in the sump, are still fish in the system. Why not get fish you want in the main display tank?