A picture would help. Once concern I have is that when rolling the tank out, the weight may be enough to shear off the shelf and its supports. What size TV was the cabinet meant to support. Another thing that comes to mind is that once you slide out the tank (if I understand your proposal correctly) the cabinet may become top heavy and tip over onto you. At an average 10lbs per gallon, you are looking at 150lbs or 68 kilograms for the full tank.
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