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I can detect phosphate, ammonia and nitrate in the skimmate. The nitrate level in the skimmate seems to be correlated to the amount in the aquarium water. I assume this means it is not concentrated but carried over. Recently, I measured ammonia between 0.5-8 ppm in foamate. Concentrating ammonia might happen through the skimming of a large organic acid salt of ammonia, though I have not investigated why an ammonium salt would exist in a sea of sodium ions. Another explanation is that the salicylate test is detecting an organic amine. Skimming also captures microalgae (skimmate fluoresces under long wavelngth UV) and bacteria. Maybe test conditions break open the cells, releasing ammonia. Since fresh skimmate also shows ammonia, it is probably not due to bacteria metabolism after the skimmate is collected though I am still not a 100% convinced. Phosphate is detected in the 0.2-2 ppm range and might be concentrated with a large basic organic molecule or comes over in the shells of phytoplankton. Support for the latter notion is that acidifying the solid in the skimmate with dilute HCl for five minutes generated twice the amount of dissolved phosphate than in the liquid. Also, the phosphate test conditions might be acidic enough to dissolve some of this solid, giving the illusion of dissolved phosphate. Overall, a skimmer removes nitrogen and phosphorous in amounts that are useful but not substantial. Given the volume of skimmate my skimmer collects per day, phosphate is reduced in the aquarium by 0.001 ppm per day.
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