My Calamondin is really not very well. It's leaves are very droopy following a period of falling off.

It is a small tree only about 24" tall, in a pot. It's not been over or under watered and the roots are okay. The tree has been removed and repotted in case it's infestation, but there was apparently none. However I notice there are still little black flies in the soil. The leaves are looking very dry and sorry for itself.

It's in good sunlight.

Any suggestions?