Gilvin, gelbstoff or whatever you want to call them are yellowing compounds usually resulting from decaying organic matter can absorb UV to some degree. The gist of that article was to examine UV transmission when adding or changing GAC. Test for gilvin by looking for yellowing of old water in a deep white container (such as a 5-gallon bucket) and compare it to a bucket filled with freshly but thoroughly mixed saltwater. Now, to expound on UV in aquaria. I wouldn't be overly concerned with excessive UV production in any commercial LED unit (at least any of those I've seen.) Metal halides are a different story. Use acrylic material to remove UV. All glass shields I've tested only marginally remove UV. Is UV good? UV-A is used by zoox in photosynthesis. UV-B absorbing compounds (Mycosporine-like amino acids or MAAs) can be produced by the shikimate pathway in all zooxanthellae species and clades, thus it shouldn't be much of an issue. UV-C is unnatural and is produced in at least small quantities by lamps containing mercury. On the flip side, when I was managing the coral farm, one of the retail guys showed me corals were colored up in a circular area within one of the 6' x 10' culture vessels and he didn't know why. As it turned out, someone had removed the acrylic shield on the MH directly above the coral. I was intrigued, and tested PAR with and without the guard and their wasn't much difference. I attributed the increased coloration to UV. I'll stop now - this is all old news.
