I have a water hyssop. It is doing well so far. What kind of food does it like? It's in full sun in a hanging basket and has doubled in size since the beginning of July. I live in northern CA. Thanks.
OK, a quick word about fertilizer before I answer your question. Fertilizer is not "feeding" plants, although the term is often used. Plants actually make their own food; you don't "feed" them the way you feed animals. They take the basic materials they need to grow and live from the air, water, and light; the minerals - supplied by soil - are necessary to hold the basic materials together. Kind of like lumber and nails -- you can't build a house of nails, but you need nails to hold the lumber together. Fertilizer adds to soil from which some of the minerals have been used up. Theoretically, if you add fresh soil, or have soil that is constantly renewed, you don't ever have to fertilize.
For your bacopa, as for most plants, a simple balanced fertilizer with essential nutrients in a ratio of 1-1-1, or 3-1-2, will provide all that's needed. Learn what these numbers mean: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-numbers-npk.htm
There is more to it than this, but this is a good place to start.
In almost all cases,