something strange in my tank

Question
My 47gal tank has been up for 10 months.I have 12 guppies the filter is a under ground and a penquin bio wheel 200 backtank.we change the water every 4 months and we drain about 75 of the water out. The pH is 6.5 there is a strange silk like cocoon with black dots in it floating in the top. Have you seen this and is it going to hurt the fish. We have removed it. Thanks for your help.

Answer
Hi Gary;

It may have been a chunk of waste or food that decayed and became covered in fungus. The fungus can grow to several times the size of the original gunk. It doesn't usually float on the top though. Perhaps it was an insect egg sac that fell in the tank from somewhere on the edge?

At Your Service;
Chris Robbins