If your plant hasn't wilted, you can eliminate under-watering as causal. It could be over-watering, or in this case it's more likely natural senescence.

Why on earth would you use banana peel tea on a plant? Was there an objective in mind?

Al

Did you recently move the plant to a new spot? Reduced light exposure can cause a plant to loose it's older leaves as it adjusts. But I do see a new leaf emerging so there may be no need for concern.

With new growth coming along, I say it's just age.

New growth isn't a clear indication that the plant is growing. True growth is measured by a plant's increase in dry weight; so, if a plant is shedding a leaf and growing a leaf, it's roughly in a static state, which is not a good place for a plant to be. I'm not necessarily saying the leaf ISN'T being shed as a result of normal senescence, only that new growth might not indicate an o/a increase in mass.

There is no better fertilizer program than one you can control to the degree you know exactly what nutrients your plant is getting, how much of each, and when nutrients will be available for uptake. There is an ideal level of nutrients in the soil and an ideal fractional amount of each in comparison to each and all of the others. Most plants use about 10X as much N as P, and about 3/5 as much K as N. The rest of the nutrients also have an ideal level determined by a function of N levels and the overall level of total dissolved solids (TDS) in the soil solution. Adding tonics, miracle concoctions, or supplements targeting an increase in 1 or 2 nutrients w/o good reason or a strong likelihood what you are doing is going to correct a deficiency, is far more likely to result in limitations than benefits.

Al

It wouldn't be static. With dieffenbachia it's the cane that has the most dry weight. With every new leaf the cane becomes taller so dry weight would still be increasing.