cobwebs in new tank

Question
I have a 6 gal eclipse tank with filter and biowheel, light and heater.  Our first three guppies died, we then changed 2/3 of the water, dechlorinated, and let it sit for two weeks before adding three new guppies. Now there is white floaty fiberlike stuff hanging off our plastic plants and small decoration. We keep our tank at 78 and use the light for a few hours a day. Our PH and everything tests normal.
What is this stuff?

Answer
Hi Patty,
Hmm...Sounds like something I've encountered a few times before in my aquariums. It is likely a common fungus that develops in our aquariums when leftover food or waste is built up. It can definitely resemble cobwebs in both color and form.  Saprolegnia fungus may be to blame.

The best thing to do is vacuum the tank well and siphon all the white stuff away. You may have to repeat this a couple of times depending upon how bad it is.

Afterwards, simply being careful not to overfeed and vacuuming often will prevent this from re-occurring.

Best of luck and I hope this helps!
Karen~