It is not necessary to purchase expensive "bonsai fertilizer" to plants it makes little difference. I simply use common water soluble fertilizer mixed at the recommended strength. Choose a balanced (10-10-10 or 20-20-20) product unless your specific plant has any unusual requirements.
Some use a high N product in the spring to get their plants off to a good start (20-10-10). There are also acidifying fertilizers ( Miracid) that are made for plants like Azaleas or many evergreens.
Potted plants in general and bonsai in particular , due to their free draining soil, require regular fertilization when they are actively growing. If your plants are healthy and you are seeing new growth, with your tropicals now, fertilize them about every two weeks. Following the directions on the package of the product of your choice.
your humidity tray will be more effective if you use a layer of gravel in the bottom. This greatly increases the surface area from which the water will evaporate.