Q.Fiddle Leaf Fig Problems
I’ve had my fiddle leaf fig for a few months now, repotted about a month ago because it appeared root bound. For the most part, the fig looks pretty healthy and is sprouting a new leaf every week! Some things are troubling me that I’d like to address before they get out of hand.
1. Clusters of small rusty brown spots on leaves, newest leaves have minor clusters as well
2. There are annoying gnats crawling in soil and flying around tree.
3. Older leaves have some dark blackish spots (they had when I bought the tree) and are dry and cracked. Also, some leaf curling and “crunchiness” around edges.
I water the tree once a week with a few cups of water to make soil evenly wet. Pot is 1/3 full with stones at bottom and topped with Miracle Grow moisture control soil.

Sounds like your Fig might be suffering from Fig Rust and/or Leaf Blight. These articles may help you identify and treat it: https://www.gardeningknowhow.com/edible/fruits/figs/figs-disease-rust.htm
https://www.gardeningknowhow.com/edible/fruits/figs/common-fig-tree-diseases.htm
The gnats that you see happily hanging around your tree are likely fungus gnats or soil gnats, and they generally don't do any harm, but the larvae of certain types can damage the plant when they feed on the roots. This article may help you get these tiny bugs under control:
https://www.gardeningknowhow.com/plant-problems/pests/insects/fungus-gnats.htm