Don't think it was me on that tip, but whomever it was, it works. I use a small 1/2 teaspoon measuring spoon to feed my fish dried foods, that prevents me from dumping in too much
/ You can then take the handle end of the teaspoon and gently stir the flake in the water for a few seconds, it will start sinking.
Something else jumps out at me, you need more of this species. Tetra are shoaling fish that live in large groups, and minimum 6 is recommended. In smaller groups, fish will be under more stress; aggressive behaviour is frequent, weakened immune system, all sorts of issues can occur. I don't know your tank size, but if there is space I would get 3 more of this species.
You can read more about this fish in our profile, click the shaded name: Black Widow Tetra (Black Widow is another "common" name for this species). Profiles of many species are under the second tab from the left in the blue bar across the top of the page. Tetra are characins.