I agree with Dan, I paint styrofoam panels black, and put them on 3 sides, and the bottom of my tanks to provide a more secure vibe for the fish, and help maintain constant temp. Mid 70s are fine for JDs, when I snorkeled with them, the waters were "not" by any means warm.
BTW, if you have a tiny scratch or cut on your hand when you put it in the tank water, you will know if you have stray electric current, the cut will tingle.
Large live bearers are good dithers, and what JDs live with in nature.
I consider a 75 gal too small for an adult oscar, but OK for an adult JD.
And by the way both oscars and JDs are omnivores, so vegetation is normal part of the diet for both.