Japanese maples are not heavy feeders but any plant trapped in a container appreciates a slow-release plant food like Osmocote. I would use the Osmocote Indoor Outdoor formula which is an all-purpose fertilizer for all plants. Just follow the instructions on the label for the size of container and do not use more than the recommended amount.  When it comes to plant food, more is not always better as too much fertilizer can burn a plant and even slow-release plant foods like Osmocote that do not burn plant foliage can encourage too much fast growth which is then more vulnerable to insect and disease and to outgrowing the container too fast. Japanese maples are excellent specimen trees to grow in pots. Keep growing, Marianne Binetti