How can I cheaply fertilize my Aerogarden?

I have an Aerogarden (a little self-contained hydroponic system) that I got as a present, currently growing basil.  I have run out of the little nutrient pellets that came with the pods I was given, and can't really afford specialized hydroponic nutrients.  If I get a generic fertilizer intended for soil-based plants, what would I need to supplement it with to make my basil tolerably happy?

If it makes a difference, I fill it with RO water, to avoid excess calcium buildup, but I live somewhere with very hard water, so if all that's really needed is, like, calcium, I can just add some tap water to the system occasionally.

2020-10-11T11:02:09Z

The problem is, a standard fertilizer for soil plants is "assuming" that the plants will get certain nutrients *from* said soil.  They don't usually include various micronutrients that are relatively abundant in soil, like calcium.

Barry2020-10-10T09:34:17Z

I would use a liquid fertiliser sparingly.