Is it ever appropriate to “overstock” a nano tank

I have a betta and a nerite snail in a 6 gallon, have had them for about 8 months. My nitrates, nitrates and ammonia always read at zero (using a proper test kit) and the aquarium is moderately planted. I do about 20% water changes every 10 days or so. I’ve been seeing slow plant growth even with 2x a week fertilization. I was thinking about potentially adding another snail to increase the bioload for the plants; would this be a bad idea? I have an oversized HOB filter.