My fish have been down for the count for the past week, my betta especially. He started with a low appetite and listlessness, but now only moves once or twice a day, stays at the bottom of the tank with labored breathing and no appetite. I saw him flashing once but now he seems just too tired. At first I thought it might've been bacterial infection, which he's been prone to having but then I spotted these little things crawling around the glass and swimming through the water column. They're extremely small, just under one millimeter long to less than half that, white or colourless, wriggling free swimmers or climbers, they sort of looked like really tiny shrimp. I saw one swimming out from between the bettas gills and have noticed up to a hundred of them, hanging about the rest of the tank. Attached is a picture of one, which I caught in a little test tube and examined under my microscope at X120 magnification with a pink-red stain and gum medium. They're extremely active swimmers, and are all over the place. They kind of look like silverfish larvae, are they a parasite?