Since there is a crack at the bottom, you are going to have to put something under the aquarium to prevent water from seeping out the bottom, or else sealing it with silicone caulk first. Using sand and gravel in the bottom is an excellent idea either way, as that allows you to be able to see if there is standing water in the tank as well as how much, and will help keep the humidity levels up.
You probably don't want the humidity levels to get too high if you are growing herbs and vegetables, since they could be susceptible to molds or fungus diseases (the spores are often present in the soil). The tank will help to keep the humidity higher than the room is, even without putting anything extra over the top.
I've only grown tropical house plants in terrariums, so don't know about growing herbs and veggies that way, but it sounds like it's definitely worth a try at least. The more plants you put in the tank, the more you will need to fertilize them. Top dressing with compost, probably monthly, should help. If you have other aquariums with fish in them, you could always use water from the fish tanks to water with which feeds them at the same time. If you suction the gravel from those tanks, then the fish emulsion also is an excellent fertilizer to feed your plants with on occasion.
Be sure to put the light on a timer so they get at least 12 hours of sun (16 may even be the correct amount - whatever they recommend for starting seeds).
I'd love to hear how this works out for you and what you decide to grow. Good luck.
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