Hi, pond size, 20,000ltrs. Excellent Oase pump and filtration system with 50 watt UV lamp. Had major heron(s) problem, as a result got fed up with being their canteen. Since then cleaned and refilled and no fish, merely ornamental plants. A year ago had a bad blanket weed issue and used Cloverleaf blanket control which saw it off, but left a greeny/brown residue on the liner and it seems to be that residue that is rising to the top in small clumps which collect together and covert to jelly in the net. Maybe the answer is a complete drain, clean and a refill, but if there was something available to neutralise, then that would be preferable. P.S. I am only surmising that the Cloverleaf started the problem!