I agree with Tiffany that you are letting the soil get too dry. Letting the soil get very dry is an effective treatment for the gnats, but not so good for your Pachira.
If you scrape off the top layer of soil so the uppermost roots are barely covered, that may remove some of the larvae. When not dealing with a gnat problem, it is best to water a Pachiara when the top half-inch of soil feels dry. Try to find a balance between that and letting the soil get dry deeper in order to discourage gnat larvae development.
Money Trees are very hardy and tolerate a lot of watering mishaps without dying. However, they also shed older lower leaves as they age even under the best of care. Over time they tend to become quite leggy and need pruning.
