So it's far, far, far too small. It's ridiculous how small it is for FOUR COMETS.
Each comet needs 40L minimum, so you'd better get yourself a larger tank asap. You need at least a 160L. This is most likely why Vic won't grow, there's no space for him to, and it might also be that he doesn't get enough food. Or that the food he gets isn't very nutritious. If you're feeding them flakes/pellets and only that, expect to have stunted goldfish! You need to give them variety: Blood worms, shrimp, krill, tubifex, brine shrimp... Buy some freeze-dried live foods, they're full of protein and the young goldfish need it to grow properly! You can feed them some frozen peas now and then that you thaw and pop out of their shells before feeding