Yellow water in an aquarium is usually caused by a build-up of compounds and wastes. Though unpleasant to look at, it is usually not harmful to fish at low levels and can be easily remedied.
If aquarium water has a yellow tint, it is possible the fish are being overfed. Try feeding the fish less for a few days and see if the water clears up. Also try using a vacuum or skimmer to remove uneaten food.
Decaying plants can cause aquarium water to turn yellow. Different types of snails or other scavenger fish may help clear this up by eating the decaying plants.
Dissolved Organic Carbons
Dissolved organic carbons (DOC) is the term used for decomposed plant and animal matter broken down enough to dissolve into water. DOC can cause aquarium water to turn yellow.
Tannin is a compound found in many tree barks and often leeches out of driftwood used in aquariums, sometimes causing a yellow discoloration of the water.
Ammonia build-up can cause aquarium water to turn yellow. Ammonia build-up can be caused by many factors, including overstocking and filter failure.