Where did you get the sargassum from? The reason I ask is because, when I lived on the Gulf Coast of Florida, I actualy set up a "local" tank with sand and sargassum. Kept a few file fish, local hermits, and a huge chunk of sargassum that I picked up offshore while fishing one day. The life on the sargassum was absolutely amazing. Tons of tiny shrimp, a few small crabs, and several sea spiders. The problem with sargussum that is floating is that it is typically already dying due to wave action and drying out, etc. I had to replace mine about every 2-3 months as it would begin to fall apart. Hope this helps.