ok. so you are saying that the water straight out of the tap and what's on your pond are virtually the same. That's good news right there. Now you need to maintain that steady.
You need to quit using things like Ammo Lock, as that's not a natural way to control your water parameters (to Old Marine's point of self balance). This will happen naturally once you improve your water quality.
It does sound like the reason you have these spikes is because you are not doing partial water changes frequently enough. And yes, the ONLY thing that will keep your ammonia numbers in control is frequent water changes.
Remember that a pond like yours is nothing more than a captured body of water. You can think of it as a toilet. You have fish in it that are peeing and pooping and you're throwing food in there and plants are decaying, etc. The smaller the pond, the faster the water quality downgrades. So, the water will build up ammonia as all this waste enters it. And you, as the human who runs the pond, has to hit the flusher so that all that ammonia and waste can exit the pond. After all, no one wants to be swimming around in poo. (Hate the bring up a bad image, but think about your own toilet at home. Consider that everyone in the household kept using it, but no one flushed it--it would not be a pretty sight. Finally everything gets real stinky and ugly and family members start getting sick, so someone finally decides to pull the flusher and eliminate the waste.) That's basically what you're doing with your 90% water change. A water change of that level is very drastic and should only be done in an emergency situation. You need to prevent emergency situations by maintaining good water quality on a weekly basis. You do little water changes constantly to refresh the water so you never need to do massive water changes at all. Less work, too.
So, your job as human keeper is to do a partial water change every week or two of at least 25-30%. I'm not sure how you are changing out the water when you do your changes, but a good muck vac or some means of getting at the bottom layer of the water and pond floor is best. That's where the water with the worst water quality sits.
Since you are dealing with tap water, you should use some kind of de-chlor product because most tap water has chlorine in it that is deadly to fish. So, you need to make sure that gets out. That should basically be the only chemical you are using to treat your water. Every time you add new water, you use the de-chlor. That's it.
If I were you, I would do weekly water changes to start since you have a very small pond and you have a water issue. I would keep testing every other day for a month straight. If your water remains stable, then you can try water changes every two weeks instead. If you see a spike in the ammonia/Ph, then you will know that weekly changes are better for your situation.
You are in CT, so in the next few months you will be shutting down your pond--but you need to keep up your changes til then.
Then, as OldMarine correctly and eloquently put it--you will reach the point where you rarely or ever have to test your water because everything is going smoothly. I, like the sarge, rarely test my water.
Ponds are very pretty to look at, but they do require work and regular maintenance. Once you get things stabilized and you are on a regular cleaning ritual, your pondkeeping will become simpler and more fun for you. And you will have no need to ply your pond with expensive chems to keep your numbers normal.
Oh, and never wash your filter media in tap water. The first thing you do is grab a 5 gallon bucket of water and scoop some of your POND water in it. You then wash all your media in thebucket with pond water. If you need to scoop more water out to refresh the bucket, that's fine. But don't use tap water as you will kill all the beneficial bacteria that has grown on your sponges and bio balls and such.
I know I've been wordy, but I hope I have been clear. Just get that de-chlor and keep up with your water changes. You will see a difference.
