New to plants, need help!

Hey everyone... I'm new to live plants and am afraid that I maybe killing my XL Amazon Sword that I bought several weeks ago. It's currently in a 125 gal with 1 7" Jack Dempsey, 1 6" Jaguar Cichlid, 1 5" Tiger Oscar, 1 5" Red Terror, 1 6" Sailfin Pleco, 1 3" Clown Pleco, and 2 4" Clown Loaches.

The tops of the leaves are forming brown spots and some of the leaves appear to be dying (Turning brown and withering away). I have a fertilizer in tablet form that I put near the roots once a week. My pH is 6.8 and other water parameters (nitrite, ammonia, nitrate) are all fine. Is this plant dying? Am I killing it? Are any of my fish killing it?

I love the look of real plants so much better than fake plants and want to add more to my tank, but don't want to just throw money away if they are just going to die. I don't know hardly anything about live plants and the more I try to research the more lost I feel. Any help whatsoever would be greatly appreciated.

ok thanks Fishorama. I was worried with my fish selection before I bought the plant, as I've heard the cichlids I have aren't the best to keep with live plants. But so far they haven't done anything to it. The oscar and jaguar will occasionally nap in the leaves of the plant but that's about it.

I figured the very large size of the plant would work in not letting them destroy it and so far it has worked... I just hope that continues. Should I be concerned about my plecos? Do they actually eat plants, or just the algae that grow on them?