Hmm, I am stumped here.
You said the pond was on day six in the initial post. Without a serious bio bacteria bump, in the form of existing media from another pond or live bacteria, I have not seen a pond cycle in one week.
Your nitrAte reading is the one that has me seriously stumped. That indicates a very established pond. Were that the case, with your ammonia reading and your nitrite reading, I would say you are horribly under filtered given your water temperature. Again, ammonia converts to nitrite which converts to nitrate.
Your GH and KH readings are good. Don't really worry about the GH going forward. The KH must be maintained to about 10 drops by your kit.
In other words, what your readings are indicating is that the pond has fully cycled but can't keep up with the bio load it has. Which puts us back to the nitrAte reading and your pond volume.
When you added the salt, how much did you add?
Last thing, you need to list your tap water readings. Complete all the tests like you did for the pond, please. The answer is in here somewhere.