Beginner Question on Bronze Wendtii and Algae

[MclarenMP]

I have a Bronze Wendtii and it has done very well up until recently (about 2-3 months) in my 20 gal tank. The plant itself does not look as full or lush as it used to, the leaves and the stalk on the outsides are starting to drop and decay, and some of the inner leaves are falling apart. There is a small bit of brownish-red algae that is covering some of the leaves, but not to the extent I would think would effect the plant. I have 2 Koi Angelfish, 1 Sterbai Cory, 1 Panda Cory, and 2 Leopard Danios. I lost my Gold Nugget Pleco a few weeks ago due to Ick. I do have a CO2 diffussion sytem, I forget what brand, but it is the one where you mix the sugar and ingredients in the grey canister and it has the diffuser with the three suction cups. Also using an air stone going very slowly on the opposite side of the tank, more for the effect than any other reason. I use the Seachem 3 pack that has the Iron enricher, carbon solution and I forget what the third one is now, about every third day. My Amozon Sword, Dwarf Grass, and Ruffled Sword are all doing fine. Was thinking about grabbing some Algae eating shrimp to try and see if it is the algae and once I find a resonably priced gold nugget pleco I will add that again. Any thoughts or advice on what is causing it or what the best way to fix it is? Thanks in advance