Liquid fish fertilizer
Quite gentle and does not burn the plant. Can be at higher price point compared to other option from personal experience. The downside is the smell, of some brands can have a strong scent.
Worm casting
Quite an available option. The only downside is that you need to mix it with the potting soil so use it when re-potting a plant. Some people add it on top of the soil so the nutritions can trail down with watering, but I did not find evidence if this is true or not.
NPK fertilizer
Quite strong and can burn your plant. but very available and affordable. if you choose to use it I suggest using extremely small amount and diluting it in water. Make sure to follow the instructions on the box.