Re: Fish problems, need help for inexperienced keeper!!
The test results look OK to me. Which tells me you have adequate bio filtering today. Ammonia and/or nitrite level tells me that.
The pond itself provides some bio filtering. Sides, inside of pump tubing, provide surface area for bacteria to convert ammonia and nitrite. Algae consumes ammonium (part of Total Ammonia) directly. Streams and waterfalls are great bio filters. So a pond has a fair amount of bio filtering before the things we think of as filters are added. So a pond can support some number of fish. Ammonia and nitrite levels tell whether true or not for a specific pond at a specific time.
Most pond plants will also consume ammonium, but not as well as algae only because algae grows faster normally and has more total mass. Potted plants would generally add more ammonia and nitrates from decay and stuff coming out of the media, than they would remove. Too close to call.
Plants and algae aren't reliable however. When an algae bloom is growing good you'll generally measure zero ammonia and nitrates, it's sucking it out of the water as fast as the pond can produce it. But when a bloom dies the decay will increase ammonia and nitrates.
Testing tells you what's going on.
The ulcers on the fish is a serious problem of course. No way to tell what the cause was. Ulcers are more of thing that happens after the problem. For example fungus can be the problem, damage skin, cause sores that get infected with bacteria which create ulcers which is what I would call what was described. Or parasites could start the process, or virus, lack of food, lack of O2, anything that stresses fish, lowers their immune system and bacteria can get a hold and then ulcers.
I don't really have any good simple advice. There are some good sites like KoiVet, that can help you work thru all the possibilities, but it's not simple at all. My best advice is to spend some time reading. It's faster and more effective for you to read and look at pictures and tell what's going on than trying to describe it in text to other people.
I normally don't recommend trying cures without having a good idea of what you're curing. More harm than good. But in this type of case the fish sound in pretty bad shape, more than one fish is involved, and a 200 gal pond is more treatable than a 5000 gal. I'd probably try something. What that something is...I really hate to guess based on some text. Even pictures aren't a big help, hard to get good pictures. This stuff is hard enough to figure out in person.
By the time things get this bad it's pretty hard to fix without a fair amount of experience imo. Hard to talk people thru it. And once bacteria eats away things like lips they won't bounce back all the way to normal. Scars and loss of tissue are often forever. So it's just a tough road even if the fish live.
What I would do, not saying you should, is maybe put down the fish that look in the toughest shape. Like once fish start losing lips or ulcers look deep or cover a lot of the fish I throw in the towel. Then I remove all muck and decaying matter from the pond. Repeat that daily or even a few times a day for a week or two. Even if you're not getting much the stirring helps break down what remains.
For a 200 gal pond a simple minnow net and just scooping around the bottom works great.
Maybe I might get an air stone going, but I seriously doubt that would help much given the pond size and you already have a pump. More of a covering all bases thing. From the pictures the fish look well fed, I'd continue. Keep testing water just to be sure nothing changes. Smaller ponds can go from good to bad fast.
I would only add that it isn't normally a high water table, but the exact same effect. A high water table can do it, but it has to be higher than the pond water level which is unusual. Normally when there's a high water table you run into that problem when digging the hole, unless the table happens to be low at the time.
More common, especially when I hear rain, is clay soil not draining.
Water is slow to drain thru clay soil so it only takes a little bit of water to get under a liner to fill all the space between liner and clay. When the water level in the pond is lower than the ground the entire pond and liner is like a boat in a lake and starts to float. Allows more water under, more floating, pretty soon the pond over flows putting more water under the liner.
A liner in a clay hole really compresses the clay. If you were to empty the pond and remove the liner and fill the hole with water it would probably whole water for a long time. It would look exactly like a high water table, but it would just be slow drainage.
Building up the sides like finnipper suggested is the easiest fix imo. The water level inside the pond just has to be even 1" above surrounding ground. Water still gets behind the liner, can't be helped, but can't get above ground level. So the water in the pond will always be heavier than the water under the liner can push up.