All right gang; let's add a little content to this thread.
Recently there has been some talk about phosphate and how to remove it from one's tank. One of the things about phosphate is that it is all over the place. It is added to tap water to inhibit corrosion of the distribution system often in varying amounts. Polyphosphates (sodium hexametaphosphate) is usually added in low doses, around 1 ppm. Orthophosphate, like zinc orthophosphate may be added at levels as high as 20 ppm. Obviously that is a major reason to only use RO/DI to make salt and topoff your tank.
Another major source of phosphate is the food you feed your reef. This type of phosphate is usually organic phosphates and you may have a lot more than you think in your tank.
The reason for this is your test kit doesn't
see it. Most test kits can only detect simple orthophosphates. They won't tell you how much poly or organic phosphate you might have in your tank. Polyphosphate can be boiled, with a little acid mixed in, and converted to orthophosphate. This is fairly easy to do. Organic phosphates are a different bird. They need to be boiled with strong acid under oxidizing conditions to be broken down to orthophosphate; a tedious and somewhat dangerous procedure.
Having had a lab I was able to check for total phosphate in tanks, that is all 3 types of phosphate. Often a tank that indicated zero phosphate with a standard test kit would have anywhere in the range of 3 to 9 ppm when boiled with sulfuric acid and potassium persulfate. A more vigorous treatment with hot nitric and perchloric acids (don't try this at home kids) would raise that range to from 3-14 ppm.
For the most part this
unseen phosphate can be utilized by algae and may explain why reefers will have alga problems in a tank that tests near zero with their test kit.
Well what is one to do?
The phosphate "sponges" will remove simple phosphates but are not very effective in removing the more complex organic phosphates. Skimming can remove these phosphates better and, depending on the contact time and skimmer efficiency, can be quite effective. Harvesting algae from a fuge or sump will help but, as Doc Ron has pointed out, you need to have quite a harvest to really make a dent in it.
The hard fact is that there is no method to fully reduce it but you can limit the amount through regular water changes. By doing so you will reach a maximum level for your tank and after about 6 months this level should remain fairly constant. In a tank where water changes are not practiced the sky's the limit as it will accumulate indefinitely. This one of the reasons I still stick with doing weekly water changes of 10-20%.
There is more discussion about this and other "Mystery Science 3000" topics over on the chemistry forum for those of you who have the
guts to venture there.