You don't say how long you have had the fish or when you last cleaned their aquarium thoroughly. The undergravel filter relies on oxygen to keep the bacteria here functioning properly, but it sounds as if a lot of dirt has accumulated in the gravel, which is being raked over in true carp-like fashion by the goldfish, searching for morsels of food. You can find more about how undergravel filters work here.

Try using a gravel cleaner, which also acts as a siphon. It will stir up the gravel in the tube, lifting the dirt out of it, and removing the contaminated water from the aquarium. This also helps to improve the efficiency of the undergravel filter itself, without the need to empty the whole aquarium and wash the gravel by hand.




As the goldfish are growing larger, so it may be helpful to add a power filter to the aquarium. There are various designs, which can be fitted either inside or outside the tank, depending on the model concerned. They operate independently from the air-pump, having a separate motor to draw water through the unit.