While diffenbachia does contain toxins (many plants do) the severity of its toxicity are greatly exaggerated. Yes, it's a bad idea to eat it or get your hands covered in sap and then go rubbing your eyes. I'd say the same thing about jalapenos, raw horseradish, onions, raw eggs or meat products, bathroom cleaners, broken light bulbs, many common aquarium fertilizers, dead fish...etc.
If you've got small pets with a habit of chewing on plants or very young kids, keep the plant out of reach--which, ideally, you'd do with *any* plant not specifically intended to be ingested. If you're pruning, clipping or otherwise handling the plant such that stems are broken--wash your hands.
Otherwise, you're free to enjoy a really great plant for the house, garden or riparium.
However, you do need more light for it in this set up. Ditto pothos, philodendren, ivys spider plant and pretty much any other plant. You can put full spectrum bulbs in your ceiling unit and make a point to keep it on more, suspend a light over the tank for the plants, increase the PAR of the tank light and suspend it high enough that it casts light on the houseplant(s) you add to the tank, set up an auxillary reading lamp or other light source that is left on specifically for this purpose--or change the curtain setup to allow more light in during the day.