Plants can outgrow their container
Unfortunately, plants are not like the fish in your tank that only grow to the size of their home. Plants grow root-bound when the amount of plant root volume exceeds the amount of soil volume in their container.
To check, gently lift out your plant from its container. If you can see mostly roots and very little soil, then it is time to transplant your plant into a larger container.
Select a container that is about 20% larger in volume than your existing pot. A good rule of thumb is not to use a container that is more than 4 inches wider and deeper than your existing root ball. Use a good potting soil and make sure that the top of the existing root ball is even with the top of the soil in the new container.
When you are done, you should have no more than 2″ of new soil surrounding all sides of the root ball. Add a little starter food, water in and you’re good to go!