I am interested to know what organic fertilizers are used by those of us who
grow trees indoors. Specifically does this promote insects like fungus gnats

I rotate among several different fertilizers year round. The organic one which
I generally use is Neptune's Harvest, which contains fish byproducts and
seaweed extract. My plants seem to like it for the most part, as long as it is
not too strong.
Fungus gnats are promoted by soggy soil and poor watering practices. I doubt
that fertilizer makes much difference.
The only problem I have found with fish fertilizer is that when you use it
outdoors it attracts yellowjackets, so be careful.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)