I definitely wouldn't use that as a stand.
For one, if your turn it sideways, the main horizontal platform where the tank sits is joined to the side of the two outermost vertical supports.
The weight of a glass tank is supported by the bottom rim of the tank. So all the weight is at the perimeter of the base of the tank. A proper tank stand for a glass aquarium is designed to support the rim of the tank. This bookshelf isn't constructed properly to take the weight of the tank and any movement. You are putting all the weight of the tank on a few bolts or screws that are in particle board. Wood on wood gives you strength in construction. Any weight on those screws and they will probably rip out of the particle board. The shelving in the middle while it may look like it would provide vertical support....probably will fail.
Please excuse the poor drawings...
This is proper construction for a tank stand. Wood on wood - same as you would build housing or decking.

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This is improper construction for a tank stand. The weight of the bottom rim is being supported by screws or bolts or cams ...not sure how your bookshelf is built. Particle board will chip, the screws bolts or cams will loosen, and your stand will fail. In the case of your bookshelf, the vertical supports aren't doing much. If the tank shifts, the screws cams or bolts will rip out of the stand and it will collapse.

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