What's happening to my plants?
For some reason the picture wasn't showing up as an image so I had to put it here as a link. I'm sorry for the reflection and phone camera quality. I hope you can at least sort of see what's going on. I planted those maybe three days ago. I planted them in carib sea flora max substrate with some sand on top of that. The big one is an amazon sword and the leaves were all green when I got it 3 days ago. The other plant next to it came at the same time and the leaves were also very green. I think it's some kind of sword plant, but I'm not really sure. Now three big leaves on the amazon sword are brown and two of the leaves on the smaller plant are brown. And there aren't many leaves on that plant to begin with! I also have a java fern in there and it's starting to do the same thing - it's almost like there are brown cracks on some of the leaves.
My tank's 55 gallons. Water's at 76 degrees - I turned my heater off because it's super hot where I am and the heater was causing the water temp to go too high. Ammonia is 0. Nitrite is 0. Nitrate is about 6. The lighting was given to me and I can't really tell what it is other than a 40 watt wide spectrum light. I'm setting up new lighting as soon as I can figure out how to hang it without drilling holes in the wall or ceiling. I'm also using flourish and flourish excel. The fish seem totally fine and are acting and eating as they usually do.
I also had a large pleco in that tank until this morning. I knew he needed a bigger tank, so I gave him to someone with a 180. Maybe he was snacking on them and that's why this happened? That doesn't seem all that likely to me. What else could be wrong? Do I have to cut off the affected leaves including yellowing leaves or can they be healed?