Chester's going to be a big male, and the most brave and mean one of the bunch. Mark my words..
What you are seeing is him asserting his dominance, it's perfectly normal and nothing to worry about. He is essentially saying "I'm bigger than you, don't get in my way, I eat first, blah blah blah." One of the wonderful things about monitors and the main reason keeping them together works even for multiple males is that they will establish a hierarchy instead of killing one another outright. The same is true for adults and babies, but the reason it is often ill-advised to put two unsocialized adults together is that until the chain of command is established, they need to be able to get well away from each other, and as adults, it's hard to provide a cage that big. Babies simply work out the chain of command as they grow, making it much easier.
Oh I know all about the messes monitors and any other reptiles will make in their water bowls. Fish and crustaceans are okay as staples for niles, and in fact they eat more of them in the wild than they do rodents. One of the main reasons rodents are a much preferred staple though is that they have fur, whereas fish and crustaceans.. Not so much, as you just found out.