The sugar won't feed the bacteria that produce nitrates (nitrates come from a set of bacteria that break down nitrItes), nor those that consume nitrates (few in the FW system, anaerobes). It may supply a few carbs for other bacteria not involved in the nitrogen cycle, but there really isn't enough in there to cause a problem. Salt is used to counteract the effects of high nitrItes because the NaCl binds with the fishes globin, while still allowing the globin to carry oxygen. Nitrites bind with the globin in the blood and prevents or reduces its ability from carrying oxygen.