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Over the weeks I've been seeing that my tank has this white stuff develop under the filter (behind the spout of water) next to the surface of the water on the tank walls. I clean it off but some two weeks later it reappears. I've never put much interest in this until something hit me in the head to ask here.
First off, what is this white stuff? Second, is it threatening, or does it ring a bell that something needs to be cleaned?

it's probably the level that the water was before, and it has evaporated down to that level. No real biggie, its just a water ring.

More than likely you have hard water and it's a buildup of the minerals. You can clean it off with a straight edge and white vinegar.. You just need to keep on top of it as it can get rather a pain if let build up too thick. You need to remember if you use the white vinegar that you need to clean the whole tank. You can use the straight edge to scrape it regardless of the water or turtles in the tank. You just need to make sure they don't get too curious and get nipped..

like the others said.. its buildup of hardness in the water. i use a flat razor to scrape it off ( i only do this with my turtle out of the tank)
it does become quite a pain. for a month (after i did a GOOD GOOD cleaning) i bought about $20 worth of bottled water... used that for my turtle and got no buildup. it worked great, but i decided it was foolish to spend that much money just for water so i could be lazy and not have to clean.. it does help though. then i filtered all the water through my britta for the next water change.. which worked almost as well.. but now im just too lazy for that too lol

It could also be a bunch of shed skin gathering or mildew/mold.

I do have those rings and I clean the tank when they really show, but that's not what I'm talking about. I guess I forgot to give detail. This white stuff looks kinda like lice, just that it's wet .

Ew. Can you take a pic?

is it like planera??? Almost like white little worms???