WHAT is this

[Planted tanks]

Cycling a 10G and never had any fish in there. Just did a water test and all parameters came out perfect so did a 50% water change and immediately noticed these. Water comes from a 30 gal brute trash can that is in the house and always has a lid on it. It was a new can and has never been used for trash so I don't think they came from there, also the pump I use to fill tanks would have probably demolished them. They are approx 1cm in length, twitch like a shrimp to move, and something I habe never seen before. Anyone have a clue?

 

[Ch4rlie]

Its a insect larvae of some kind, possibly mosquito larvae.

If you have fish with large enough mouths, they will deal with most mosquito larvae otherwise suck the larvae out with the syphon hose.