Added about 1/3 of the water today and the cap is holding so far... no water problems. I haven't started planting yet though, so I imagine that will stir a little muck up but it shouldn't be too bad.
I'd suggest you drain down the tank to the CAP. Better drain bellow the cap. To do that use you syphon hose in a back corner.
Then plant and fill the tank. More work yes, but your tank will be much cleaner after the planting.