I don't know if an aquarium is the best place for cacti. I'd be afraid that they'd stay moist too long after you watered them or it'd get too humid. It'd also be hard to get enough light for them. During the winter they don't need as much but during summer when they're growing they need full sun. If you like succulents though I could see haworthia, gasteria, and other similar succulents doing okay in a terrarium. They don't require so much light and can handle being moist longer. You'd still have to leave the lid off to keep it from getting too humid and preventing their pots from drying out though.