what are these? and are they harmfull?

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How can you not see anything? They are fairly obviously flat worms. Most flatworms are harmless, some fish eat them. They are a "pest" because if you over feed your tanks, or have to much excess "nutrients" they can multiply like wildfire, and since they aren't exactly attractive they can be very unpleasant in large numbers. To keep their numbers down, try not to over feed, and keep your nitrates, phosphates, etc, as low as possible if not 0.
 
QUOTE=Kevin8888;5756273]How can you not see anything? They are fairly obviously flat worms. Most flatworms are harmless, some fish eat them. They are a "pest" because if you over feed your tanks, or have to much excess "nutrients" they can multiply like wildfire, and since they aren't exactly attractive they can be very unpleasant in large numbers. To keep their numbers down, try not to over feed, and keep your nitrates, phosphates, etc, as low as possible if not 0.[/QUOTE]

Really? To me that picture is horrible so I asked if he could provide a better one.. I'm glad you have an eye for that type of thing.. To me they could have been anything on his glass. In the link below you can clearly see a flatwrom. In that pic all I see is blurry spots on glass. That's how I can't see what they are..

http://www.monsterfishkeepers.com/forums/showthread.php?398447-Flatworms[
 
Well the picture is also blurry to me, I find it fairly obvious that it is the most common veriety of flatworms one of the Acoel Flatworms, I can tell by their shape. The one you linked is predatory, an interesting specimin, but not something I'd want in my display, these ones, unless I'm mistaken... I will see if I can post a pic correctly... are generally considered harmless.
Capture.PNG
It may not be the "exact" same veriety, but as long as they arn't huge, or brightly coloured they are generally safe.

Capture.PNG
 
Also, if you want to remove them, you can blast them off the rocks/glass with a "turkey baster" then use it to suck them up and dump them down the drain.
 
Yeah, I wasn't aware that they had ones that take a particular shape. The only ones I have had are the ones that I linked and those things will take any shape.. I have only found 3 so far since that posting and each time they get pulled out right away.
 
Yeah, I wasn't aware that they had ones that take a particular shape. The only ones I have had are the ones that I linked and those things will take any shape.. I have only found 3 so far since that posting and each time they get pulled out right away.

Most of the Acoel worms have that general shape, some are colourful, some have a 3rd "tail" like projection between the other two. its harder to see the shape when they are on a surface or feeding, as they "morph" a little bit, but when swimming it is pretty obvious. I actaully really love some flat worms some are quite beautiful, sadly most of the big/beautiful ones are predatory so I cant warrent keeping them if I find them, I cant even set up a small tank for them as they usualy have specific foods they eat but what those foods are are hard to figure out.
 
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