In the past 2 or 3 weeks I noticed some sort of fluffy thingies floating - every now and then - inside the tank. Originally, I thought it was some leftovers from the plants, but after I managed to pick one up, it was actually yellowish in colour and slimy. It looked and felt like bad quality jelly. Is this some kind of fish waste I'm not aware of? Or bacteria? It seems benign, though.