Cane, it is indeed down to species. Most tetras are mainly insectivores so they need some protein in their diet, like the occasional meal of bloodworms or daphnia. Some plecs are vegetarians, some need wood in their diet, others are meateaters. Barbs are often herbivores but will eat live foods. Livebearers are omnivores.
I am not so sure that bread is a good idea: surely it is likely to contain saturated fats=bad for most fish? The Tropical Fishlopedia warns against bread for this reason, and I have a feeling they might have a point where most common tropical fish are concerned.
What I do is feed most of my fish one small meal of flakes/tablets every day and one small meal of either veg or TetraDelica (3 kinds)+ one fasting day.