How do you fertilize the tank?
Well, with no fish in it anymore, it's not getting ammonia/nitrates from food and waste and I've only been topping off the water not even doing changes so there really hasn't been anything added that could be considered fertilizer (natural or synthetic) in many months.
The plants are getting all their nutrients from the substrate/soil I assume, the water column must be pretty much devoid of nutrient as a large part of the tank (20-30%) is filled with H.difformis which AFAIK is a bit of a nutrient sponge. The only thing I've done recently (two months+ ago) was add a 2-3 ml of Seachem Flourish (traces) and some Seachem Equilibrium, maybe a 1/2 tsp.
This tank is a testament to the amount of "neglect" an El Natural, Walstad, Soil based (call it what you want) type of tank can endure. I almost feel bad, but it's become a bit of a science experiment, waiting to see when things will stop growing. That said, it really has slowed it's growth, but it's still steadily producing a good crop of plantlets and cuttings every 2-3 months.
Just like nature, it seems to do just fine when left alone.