We have heard much about Crop Circles in the past few years where mischievious folks leaving the local pub stamp various mystical designs in farm fields. I have Tank Circles. Mysterious little spots that have sporadically appeared on the glass of my community tank on occasion. They appear overnight. Small (about 2-3 mm diameter) light brown in color and in clusters of from six to perhaps 40 or 50. This morning I found a small cluster with one of the platys trying unsuccsessfully to eat them. I watched her vainly nipping for awhile then left for 10 minutes and when I returned they were gone. Previous ones have stayed until I wiped them off. I once wondered if these might be eggs but the community has platys, mollies, corys and neons. I doubt that any of these has such peculiar reproductive habits. Flat and round in shape, I don't think it is algae either. Yes, the tank is clean and clear and healthy. Not harmful, but curious. Can anyone explain this?
