Such a thing as an invisible spider in my tank making a web???

[Delphinus]

Odds are pretty good we're talking about a vermetid snail (stationary). Look for a tubeworm like growth on the rock near to where the web comes from. Try a search on "vermetid" and see how it compares.

They'll spread like mad in a tank (you think "one" of them drives you crazy ... ). You can epoxy over them or try jabbing them with a screwdriver or something... or snip them off with side cutters but be sure to cut as close to the base as possible as they can retreat a fair bit into their tube and if you don't get the snail itself then all you do is give it a haircut (so to speak).