Clear 'plastic-like' floating sheets in water?

I recently cleaned out (with only water) an old 30 gallon setup, filled it with fresh water, large gravel (from previous setup), etc. I placed a new 30 gallon external mesh-type flow-through filter box on the tank, and approx. 4 days later, I noticed a 'clear, plastic-like substance' floating in the water. It seems to be growing, but that could be my imagination? It is difficult to pick up with a net, and seems to sieve through, breaking into smaller pieces when I attempt to remove it from the aquarium. I cannot find anything on the net, or in my resources, and have never before witnessed such an occurrence or water problem. I did allow the previous setup to sit stagnant for about 3 months, before finally dumping it, cleaning it, and re-filling it. After refilling with new water, I placed an appropriate amount of dissolving-type tablets in the tank, to remove chlorine, add slime, etc. I am at a loss as to what this might be, any ideas? Thanks for your time, and any help or ideas, too. I've painted fish for many years now (www.ipaintfish.com), and have simply played around with tanks in my classrooms (I also teach) ... but I've never ever seen anything like this, weird. Thanks again! 'Jason :coffee2: