Sure, there's the possibility, but really no more than with commercially grown plants, which can have pests AND pesticide residues. I've been using native plants for some 15 years now, and never had a problem with introduced mites or anything like that. I HAVE, however, inadvertantly introduced aphids, snails, leeches, predatory beetles, and hydras to my tanks with pet store bought and nursery bought plants. As for toxic plants, don't worry about it. The only possible plants I can think of that might be a problem would be something along the lines of poison oak or poison ivy, but these don't grow in the correct habitat anyway.