there shouldn't be any food left over so it sounds like you're feeding them a bit too much. in the wild fish often live on the brink of starvation and can easily go 2 weeks without food, their whole digestive system is not set up for getting lots of food every day so it's much better to underfeed than overfeed.
you should be using a mix of prepared good quality flake/pellet type foods supplemented as a bit of treat/variety once a week or so of some meaty frozen/fresh foods like bloodworms, don't give them too often but it's good to use them once in a while. Also some fresh veg such as peas and cucmber will be happily taken by most fish. Some such as plecs will need fresh veg regularly. Getting the right mix of foods is fairly important to your fish.
Give them a starve day once a week to reduce the risk of constipation, otherwise feed them a small amount which they can eat in a few minutes. It should be a feeding frenzy when you put some food in, if it isn't then you're feeding too often.