well...first the Xenia probaly died just because they don't travel well. Although they are very hardy once established they have to be carefully packaged and acclimated.

the flame scallop which sadly are sold just about everywhere, rarely do well in the home aquarium. There is not enough known about them to properly house them long time...the biggest thing is what they eat in the wild...the snail just started eating once the "death smell" started coming from the dieng scallop....nassaruis have an outstanding sense of smell....within seconds of me feeding my fish the snails eerily rise out of my sandbed to find the food.

So I wouldn't worry about your nano as long as your parameters are good as you stated.

HTH