My red eared slider turtles are going nuts?

I have 3 red eared sliders (used to have 4, it's a very sad story, tell me if you want to hear it) and they won't stop moving. mostly they swim against the gl, but sometimes they pile on top of each other and slam onto the bottom of the tank. We just cleaned the tank but it gets horribly dirty in less than 1 week because the turtles keep stirring up the "bad stuff" on the bottom of the tank. All day and all night I hear them slamming against the tank and stirring up the pebbles. you might think that the sound of rattling gravel is calming and natural, but if it is *this* loud and *this* frequent, it gets *this* irritating. Literally, they won't stop moving for 10 seconds. When I get near them, they "stand up" and pound against the gl even more. The tank is beginning to crack on the side and I can't find peace and quiet anywhere downstairs. I can even hear the turtles when I'm playing a computer game upstairs with the volume turned up. Just recently, today, I have not even heard the rattling stop for 3 seconds and the tank is extremely dirty. They did not even do this a few months ago. I think that the turtles go even crazier when I approach because they are so hungry. They probably burn tons of energy like this. We bought a new large canister of turtle food less than 3 months ago, and it's already half gone. I can't even think about adding more gravel because they kick it aside into a pile within 2 minutes. I'm really worried about them, they are burning energy faster than they can eat, and I don't want any more to die. The tank gets so wavy from their motion that it looks like there is a tsunami inside. They keep me up because the rattling pebbles are so loud. Their shells banging against the tank are causing damage to their own shells and the tank. The rattling and banging are so disruptive that I don't even want my friend to come over anymore. please help, I'm worried for them