Can you possibly post larger pictures? When I click to view, they don't work.

Bigger pictures would help.

I know things aren't done completely properly with my pond.
No pond is run completely properly.

As to the cause...if these are sores (ulcers) from bacteria the cause is sure to be from many different reasons. More bacteria really isn't a likely cause, normally it the fish not being able to fight off bacteria. We come in contact with harmful, flesh eating bacteria all the time but we have defenses so normally no big problem. But if our immune system isn't working too good the bacteria can win. Same for fish.

So the best cure is always the same, improved pond keeping. Testing water for ammonia and KH at least. Keeping the bottom clean (for your type of pond). Making sure water changes are done properly (24/7 drip is the easiest and safest). Good food...etc. That keeps their immune system as healthy as it can and gives them the best chance to recover and/or not get into trouble.

I don't like to suggest people use Melafix because anyone asking these types of questions probably isn't at a level needed to use these things properly. It's a Catch-22. If you know how to use Melafix then you don't need to ask if you should use it. Not saying this to be mean. Just see a lot (almost every) person who posts this type of question start throwing cure after cure at their fish and that stresses the fish even more and that reduces immune system. And it doesn't fix the cause. So imo it's much better to spend the time and money on things that will help longer term. Test kit and time learning. You may lose these fish, but if that's going to happen it's unlikely that will change by adding some Melafix.

If you do want to add Melafix or any cure you should read the directions carefully and follow them. You should know the pond's volume first. Most people don't. You should test ammonia first no matter what you do just to know the state of the water.