Dirty gravel can make a tank cloudy or buildup and kill fish by the harsh ammonia and nitrates. But a simple thing as bringing a goldfish into a tank, like a comet goldfish, can easily do the trick. What's weird about goldfish is that they are scavengers. The reason why it's weird is that goldfish are considered top level swimmers due to their upward mouth. But yet they spend most of their days at the bottom of the tank eating unwanted food and wastes... Even algae! So if you can't keep up with the wastes on the gravel, either stop feeding as much or get a goldfish! Beautiful and their seem to be bottomless stomachs make them an excellent pet.