Root tabs are pretty much the cheapest ferts you can get, and since crypts are root feeders, a really good way to see them over a long period of time. People sell capsules full of Osmocote+ for pretty cheap, maybe look into those? You'd only need one every 1-2 months. Other than that, upgrading the light (use a desk lamp if you have a spare one) would be good too. I've heard that liquid co2 can make some crypts stop growing, so if you do use it, dose very lightly.
Yes, I know you said improvements without spending money. The thing is, the more you put into caring for plants, the more you get out. It doesn't have to be expensive, if you just plan things out and buy smart