That's probably it. The stocking density is what determines how much you feed, and then overfeeding is icing on the cake. Nitrogenous waste enters the tank through feeding. I would consider the feeding schedule you described as "power-feeding", which is typically okay with smaller fish since they convert more of their feed into body weight than an older fish would. Do you necessarily need to feed them that much? It's tough to say. Do they consume all of the food within 5 minutes or so? If not, feed less. Also, you are feeding very meaty things to them. I might try to replace some of the worms with some formulated feed such as flakes or pellets to ensure they get a well-rounded diet. The African cichlids would probably appreciate more plant protein in their diet, depending on the species.
The most common ways to lower your nitrate in the aquarium hobby are removing it with water changes, or reducing the input by not feeding more than you should. It depends what stage of growth your fish are in, but in theory feeding once per day (half as often) will cut your nitrates in half!
