Pustulio demands your attention!

Well, overfeeding is one thing. It depends on filtration system, what you're feeding, how much you're feeding, etc.
I'm the Fish Department manager at a Petland nearby, so I maintain the tanks and, my baby, our 95 gallon display planted tank. Now mind you it's 95 gallons, but on my shifts, I'll feed them anywhere between 12-15 times a day. The matter is feeding lots of small feedings rather than one big one. Out in the wild, they'll pick at some algae here, nibble a snail there and mother nature doesn't just dump in a whole lotta fish food all at once.
Also, tanks get dirty when your fish are on a cheap diet (like Tetra foods). They add ash and phosphorus to the food (which is the equivalent of chicken by product meal and corn in canine and feline diets), which pass right through the fish and fuel BAD algae growth.