Lots of things could be going on. First I assume the new 1500 pool is separate from the other pond, no water exchange?
And I don't know how long the new pond as been running.
18 feeder goldfish in 1500 gal would not be my first suspect in raising ammonia levels. If it were 1800 goldfish that would be different.
My first guess would be the test. Test some well water. If you're seeing ammonia in that I wouldn't trust the test. You can also take a sample to a local pet store for a separate test although these can be unreliable too.
My second guess would be the Pond Prime not working. They say it has something to removes ammonia. That's nice. What they don't say is how much ammonia it removes and under what conditions. So how would a person ever be able to use such a product in any kind of logical fashion?
I think if a ponder wants to play with water chemistry they really have to have a good understanding of the complexities. Here's good starter info on
pond water chemistry. The second part is knowing what you're putting into a pond. For example I couldn't easily find any info on what's in Pond Prime, so I have no idea how it works. I'd have no idea if some amount is too little or too much. And that's typical of all such products. They're sold to hobbyist to sound like they solve a problem. Maybe they work, maybe they don't. There's certainly no over sight. They're free to make any claim they like. And out of the thousands of ponders out there how many actually test the results? Most people dump stuff into the water and if the fish don't die the stuff is deemed to have worked and they tell other their friends to use it too.
At the minimum you have to consider all the minerals already in the water and how they interact with chemical you're adding, Water temperature and pH play a role too. O2 levels and time of day. Personally, for me, this stuff is way beyond my ability or desire. I don't want to run a water treatment plant, I'm not a chemist. So for me I would never try to chemically remove ammonia or even chlorine because I don't have the understanding. The most I do is buffer pH with baking soda. It's one thing, not a whole bunch of different chemicals so it's easier to understand. Testing buffer is really easy and going a bit high is no problem. That I can handle.
I prefer letting time and natural processes do their thing. It's safe and butt simple. For example, you don't have to put feeder goldfish into a new pond to start Nitrosomonas bacteria. Dumping in some ammonia into a new pond with no fish or other creatures will feed the bacteria. Regular ammonia from the drug store. It's the same stuff coming out of the south end of north swimming fish. It will work faster too because to can control the amount added and not wait for fish. Probably cheaper too. You can speed up the process by moving rocks or plants from the older pond into the new pond to add bacteria. Because bacteria reproduce so fast this only saves a day or two, but some people like to be active in the process. The bacteria is everywhere so seeding isn't needed, just saves a little time.
And then I like to add plants or whatever before fish. Wait a month, or a few months before adding fish. We think of fish as being the main dish in a pond but in many cases they're not the main source of issues.
One thing that kind of stood out to me is that you added the 18 feeder goldfish to start the bacterial process. Well, if they are actually the source of the ammonia I'd have to say you are getting the results you wanted, ammonia. To chemically remove the ammonia would seem counter productive. Now you have to wait a few days for the bacteria population to grow. That should not take more than a week. 18 feeders in 1500 gal wouldn't even come close to needing a filter or even a pump to handle ammonia. If ammonia levels stay > 0 beyond a week I'd be pretty sure it wasn't the fish and start looking for other reasons.
The feeders may well die a slow death in this process. That is understood when fish are used just to produce ammonia. Many people who use feeders in this way remove and kill the fish when the process is complete. A needless cruelty but I'm not going to throw any stones as I sit here munching on a fish sandwich.
My advice is to stop doing stuff. If you're asking general chemical questions in a forum, questions that should at a minimum be answered on the product's label, it's a sure sign you're playing Russian Roulette. On the other hand, you already have the Pond Prime, the feeder fish were always to be sacrificed, so why not dump in the whole bottle? What's the harm? And you'd get to test the results and hopefully tell us all.
