please get the gravel out of the tank. I know you don't think that it is a problem but you are wrong, for lack of a better word. get some river rock from home depot to use in the water and I wouldn't use any gravel/rock on the land side as a drainage layer. you are taking a big risk that can be easily solved.
besides the river rock can't be sucked up into a wet-vac and the gravel holds dirt. the water will never be clean because dirt always in the gravel. the river rock doesn't have that problem.