If that works, I've done something useful today
My recommendation attempts to fix the basic issues, which I believe might be the flow. 55g is a 4'-long tank. My experience with your filter: the flow is overrated; it clogs up too fast; the long spray bar I see in your tank makes the flow even weaker.
You can dump all the fertilizers you can think of in the tank, but if they do not reach the plants they do no good. That's the reason for the order of my steps: flow -> organics -> fertilization.
If none of the above helps, we can dive in deeper.