Answer: I think there might be two problems with using an aquarium as a planter. First, you'd need to drill some drainage holes in the bottom so excess water could drain out. If water collects on the bottom, even if you put a layer of gravel there, the water will sour and the soil will remain too wet for most plant roots. Secondly, plant roots don't like to be exposed to light, so you'd need to cover the glass on the inside to prevent light from penetrating. Third, the aquarium has rather thin glass which could break - and it might absorb too much heat, which can literally cook the roots of your plants. If you really want to use it outdoors, why not fill it with water and float some water hyacinths and water lettuce in it? You may even be able to find a miniature water lily. It would make quite a conversation piece!
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