One of the main downfalls about caring for a goldfish is the growth of algae that appears in the aquariums. Algae is that green gunge that grows inside the tank, usually as a green layer on the glass, this can happen mush faster if your tank is getting a lot of sunlight. It is not harmful to your fish, but it really does take away the visual appeal of a goldfish tank / aquarium, if left it will get so bad that it will cover the entire glass on the fish tank.
A little Algae is good
Whilst a lot of algae is not pretty it has a lot of benefits, top breeders who are more concerned about the health of the fish than the appearance of the aquarium will not remove the algae as it is in fact a food which the fish will graze on and is good for them providing many nutrients. The other benefit is it can act as an alarm to the condition of the water in the tank, algae is a lot more sensitive to the condition of the water than the goldfish thus allowing you to spot a change in the water before it starts to affect the fish.
Removing Algae from the Tank
You can buy tools in the pet shop that are similar to a scraping tool, like a razor blade on a stick, you use this to scrape off the algae from the inside of the tank. Also available are cleaning magnets, one magnet sits on the outside of the tank, the other on the inside that has a special abrasive pad on one side. The magnets are held together through the tank and the outside one is dragged around, moving the inside magnet with it, thus cleaning the tank.
Signs of Goldfish Illness