Only 16 inches, I would put an air stone in your tap water bucket and let it roll a few hours then change water. A spare pump will work too.
Another trick, and one that I use, is I dose my water change water to the totals I want then change water. This way (front loading) keeps the tank always at my target numbers no matter how little or how much water I change. I do dose micros separately though, but that's not possible if using an all in one fertilizer.
1st goals, dose the extra and keep at it that you have started doing. Do good size water changes and keep the substrate free of any mulm and such. Clean filter once a month and that should be just a gentle sponge squeeze in dechlorinated water. Do this for a few months and see what happens, only watch new growth and make sure it stays free of algae. If there's no improvements, then other changes should be made. 90% of problems is underdosing and not keeping the tank clean and fresh.