sick angel fish... please help.

I'm new to the forum and am quite distressed that my angel fish is sick. He is usually very mellow and hangs out around the tank, never attacking the other fish. He is in a 30 gallon tank with a balloon molly, a regular molly and 4 neons (which he doesn't bother). He has a 2-3" body with fins that make him about 5-6" tall while all the other fish are maybe an inch at max, the neons probably half an inch. I've had the angel fish for 3 years with no problems and he has lived with all of the other fish for 6 months with the exception of 2 yellow neons that were added within this last month. They are in a 30gallon tank with a Penguin 350 Biowheel filter. The tanks temperature is 76 degrees, my Live NH3 meter says it is at 0, and I use Tetra EasyBalance Nitraban. I feed him a diet of TetraMin flakes, granules and Baby Shrimp treats 2-3 times a day. They have a circular airstone and a live plant (that doesn't appear to be doing so well either... it has yellowing?).

My angel fish is in one place, leaned up against a statue upside down gently moving his fins but seems to be breathing heavily. He has no tattering, his scales are shiny, and he hasn't dulled any in color (he is a bright orange)
I did a 50% water change using a gravel vac and added fresh water and chlorine remover.
I'm sorry this is so much information, I just wanted to be as complete as possible in hopes of someone being able to help.

Any idea what is wrong with my angel fish? Is there anything I can do to help him?

His top fin appears to be getting dark close to his body and spreading up the fin. Also, im a large patch on each side it looks like his first layer of scales has air under it (like a peal from a sunburn).

I'm headed to the store now to buy a heater, as it looks like maybe my tank is too cold for him? Though he has always had it at this temperature... and has done fine for the last 3 years.

Anyone have any suggestions as to anything else I can do for him?

Angelfish need to live in water that is at least 80 or 82 degrees. They can survive in cooler water, but the stress will eventually make them sick. They also like clean water, so a 20 or 25 percent partial water change should be done every week. You may also want to consider using a water conditioner like Stress Coat rather than just a simple chlorine remover. Of course, when things are perfect for the angelfish, they will be less than perfect for the others. Angelfish will do better in a single species tank, and when they are ready to spawn, it's one pair to one aquarium. They're a real pain, but they are also fascinating and exotic. Angelfish can undergo color changes when their health changes or when they are spawning. The patchy spot on the side of the angelfish could be a parasite. A fish under stress is more likely than a healthy one to be attacked by a disease or parasite.