Hey, yellow water...it's kinda like yellow snow, isn't it?
Seriously, that isn't far off, as ammonia is broken down and nitrites are broken down, and uneaten food decays etc...dissolved organic compounds are produced. These compounds turn the water yellow. I do a water change once per week of approx 5g in a 20L tank, I am surprised every week by how yellow the water I pull out is, and how much clearer the tank is after the water change. If you are not changing the enough water to keep the water from turning yellow, you can use activated carbon to clear the water, as well as protien skimming. The carbon should only be used for a couple of days per month, IMO
, because after that, it is exhausted and becomes a biological filter, rather than removing the DOC