Tetras are carnivores (generally) and appreciate anything made from small invertebrates. I find fish look best fed on a mix of frozen and flake foods rather than just flake. Frozen bloodworms in particular are ideal. I also like to sue frozen lobster eggs (buy from a marine fish store); they are greedily eaten by most fish and are rich in fat and protein, making them useful for getting recently-purchased fish into good condition. When buying flake food, one thing to remember is to get small tubs if you have small fish. A tub of flake food "goes bad" after 2-3 months, and loses its vitamin content. I like to alternate between brands each time I buy more food: this stops the fish getting bored. You might even want to have two brands on the go at once, perhaps one staple food and one colour enhancing.
Plecs are herbivores, though some species like a bit of meat in their diet as well. Assuming you have the generic plecs like Ancistrus and Hypostomus, then slices of courgette (zucchini) make an excellent staple. Blanched (or briefly microwaved) lettuce leaves are also useful, as are crushed frozen peas. Some plecs will also enjoy slices of potato. Basically anything soft and veggie -- melon rind, carrot, spinach. Try a little of whatever you have, weighted down or tied to a stone. There are special plec pellets available, and these are also useful, but you don't need to feed them every single night, or they won't eat the algae in your tank! Once a week put a small bit of frozen prawn or mussel in the tank. Many plecs like a bit of meat. Alternatively, let them have some frozen bloodworms.
Some of the fancy plecs are more picky. Otocinclus need lots of green algae, while Panaque must have bogwood and shouldn't be fed any meat at all. It's actually a good idea to have wood in the tank for all plecs; while only Panaque actually digest it, the others seem to use it as a source of fibre.
Cheers,
Neale