I am new but have a tried and true method to defeat Staghorn, which I've encountered in 3 lo-tech tanks.
2) Do 50% WC, including deep vac of mulm in gravel if possible
3) While tank is drained and plants exposed, spray consumer H202 directly on affected plant
4) Wait 5 minutes, refill tank, adding 1tsp aquarium salt per 10 gallons, salt shocks algae
5) Immediately run UV filter for one night to kill spores and microrganisms kicked up in mulm
6) Drop in 1 siamese algae eater and 1 bunch of hornwort for every 20 gallons
7) Decreasing light never worked for me. INCREASING light seems to work for me!
In one week you will staghorn will be gone. Never lost a fish and almost 100% success rate. Algae, being a simpler organism than your plants, gets its butt kicked by altered ph from salt, nutrient absorbing hornwort, and the sensual lips of the algae eater.
I have friend who incrementally increases PH over the course of a week or two until algae bites the dust. It is risky, but the simpler life forms are more susceptible and can't adapt as easily, hence they die first.