Add a general fert like Miracle Grow etc about once every week or two.
You can spray it on the plant itself in diluted concentration(see label
instructions).

If you add too much and there is not much plant coverage or biomass,
you can get green water or you may hurt the fish if you over dose the
NH4.

I would think 1/4 teaspoon of the fert would do fine.

Regards,
Tom Barr