Hi
From the image you have provided it is a nymph and probably of the order Plecoptera Commonly called a stonefly
The immature form are aquatic and resemble the adults with the exception of gills.
Larvae are either primarily herbivoredetritivores, insectivores, or omnivores, and in some cases their diet shifts among these categories as they develop.
the most common form is herbivoredetritivores and would probably be feeding on plant and decaying plant matter in the aquarium. It is unlikely that the aduls would lay there eggs in the tank if there was no food source of this type to provide sustinance to their young.
I would need to know what you have in your tank (possible food source) and preferably a photo of and adult if possible.
I would not have thought this to be a problem in a fish tank and is probably a compliment to your skills as they only lay there eggs in slow moving, well oxygenated water.
I doubt that its a dragonfly nymph as the aantennae is quite large and visible in your photo, although they do have antennae they are usually small and difficult to see with the naked eye.