Marine aquariums nowadays have high quality seawater, or they should do if stocking is not overdone, they are properly equipped and routine seawater changes are undertaken.
The protein skimmer has advanced enormously since the first products years ago. The skimmer then was a tube with a cup at the top. An air pump drove bubbles into the tube which rose up and drove the seawater up as well. In other words, the bubbles and seawater moved in the same direction at more or less the same speed. It worked, but the arrival of the counter current skimmer was a big improvement. As the name implies, the seawater moved in the opposite direction to the bubbles giving a longer contact time and therefore more efficiency. The counter current skimmer still employed an air pump and air stone (actually a wooden block as the bubbles produced were smaller).
Now there is quite a large choice of skimmers. They can be hang-on or stand-alone types and the aquarist simply picks the most convenient for his/her aquarium. Though there are other designs, most in use today are ‘venturi’ models.
So what has efficient skimming got to do with yellow seawater? It is simply that the ‘problem’ is nowhere near as prevalent as it once was, though it still does happen. This is because of the efficiency of modern skimmers in the removal of dissolved organics.
The yellowing of seawater is caused by what is called ‘gilvin’, and this is primarily phenol compounds and carbohydrates*. To be honest, I have no idea why this word ‘gilvin’ came to be used; maybe it is loosely or directly connected to some scientific term.
Livestock produce ammonia which is an inorganic waste and processed by bacteria in the bio-filter. Organic wastes are produced as well, which can be the result of feeding, which is feces, the breakdown of uneaten food, the rotting of dead fish etc, or the breakdown of algae. Efficient seawater movement can raise a fair amount of this material into the seawater when it hopefully will be filtered out by a dedicated mechanical filter, if used. Even if it is filtered out, it is still in direct contact with the seawater, and this is the reason why mechanical media requires regular cleaning. Also, of course, the flow rate would reduce as the media clogged.
Bacteria set to work on organic waste types as mentioned in the previous paragraph. When the protein skimmer removes dissolved organics they are isolated from the aquarium, which means that the bacteria do not have the opportunity to work on them. (This is the advantage of the protein skimmer above other filters, the ability to isolate considerable amounts of organics (if available of course, hopefully if husbandry is good there will not be that much)). Once the bacteria have processed the organics the end products, or maybe some of them, could turn the seawater yellow. This is not a strong yellow; the normal result is that the seawater has a tinge of yellow to it.
Sometimes the yellow tinge is not that noticeable but the aquarist feels that the seawater, though not cloudy, doesn’t look quite right. The test is to place a small white saucer or plate inside the aquarium against an end glass panel, or fasten a piece of white paper on the outside of the end panel. Then look at the white colour from the other end- is it still white?
If it is found that there is a yellow hint then there isn’t any real need for alarm as the ‘gilvin’ is generally harmless. However, it shouldn’t be there and getting rid of it is easy. Obtain some activated carbon suitable for marine use and place it in a suitable container where there is good seawater flow, such as in a canister filter. This will remove the yellow tinge over a short period. The carbon need not be left in place, but removed and discarded once the need is gone. If carbon is used continually, remember its life is finite and it needs periodically renewing. Also, as with a protein skimmer, it removes trace elements from the seawater which are better left.
As said, it is unlikely in a modern well designed aquarium which is well cared for that the seawater will ever show any signs of yellowing. Carry out partial seawater changes, use an efficient properly sized protein skimmer and regularly clean it to maintain efficiency, remove as much detritus as possible, and don’t overfeed.
(* Reference: Baensch ‘Marine Atlas.’ Helmut Debelius & Hans A. Baensch.)