Really depends on how serious you want to be about the situation.
Once your fish show symptoms of something wrong, saving the fish might be too late if not corrected extremely quick then all you can do is save the fish that appear to be doing ok before they get sick.
IOW, what are OK or "safe" numbers for me to shoot for? And when should I intervene?
All depends on who ya ask. Generally you should never be registering any ammonia; if you do register something, it should disappear within just a few days.
California Koi Club - Ammonia calculator and an idea of what they think where ammonia should reside.
Bacteria feed very fast. The temperature and oxygen saturation and pH of the water passing by the bacteria colony is what determines the effectiveness of the bacteria. When I was doing a fishless cycling of my waters, the bacteria colonies literally droped 3ppm of Ammonia to 0ppm in less than 24 hours. One mission of bio-filters are to remove ammonia. Registering any ammonia, that is not removed completely in less than a week, then there are problems with the filtering or water's ecosystem.
Careful with the oyster shells and high pH water changes. Any pH increase noticeably increases the conversion from safe ammonia to toxic ammonia. Basic ammonia tests combine both safe and toxic ammonia in the same test. So you might be just fine with .3ppm Ammonia on a basic test since 85% of that Ammonia will be safe and your fish will be ok if your pH is around or below ~7.9. Anything above this will increase the conversion of this safe ammonia to be toxic to your fish.
SeaChem makes a liquid drop test if you are curious about how much toxic ammonia (NH3 free ammonia) you have in your water.
And I did wonder about the water changes, at least during this new-pond stage. I kept thinking that if I eliminate all the ammonia, I was defeating the purpose of establishing my bacteria. No wonder my plants are just sorta hanging-in-there....
Be careful with water changes. If your source water's pH is not much higher than your pond, then it'll be ok; otherwise, I would dump in an ammonia binder chemical to stay on the safe side before introducing the high pH source water for a water change.
My understanding is that water changes are only done in reaction to a problem or to reduce particular nutrients that are very tough, sometimes improbable, to remove naturally.
The Ammonia-to-Nitrite bacteria form very quick. The Nitrite-to-Nitrate bacteria are much more fragile; this bacteria is very sensitive to pH, water temperature, and sensitive to light exposure.
BTW, my water has been crystal clear for several weeks. Only had a brief algae bloom during the first week, then had the cola-water for weeks after that. Loving to be able to SEE my fish, and don't want to jinx myself, but can I expect another algae bloom at some point before pond has "cycled?"
Algae is prehistoric and most simple form of aquatic life with one mission to filter water. There are even naturally occuring algae and bacteria species that feed on man made synthetic oils.
Many algae species out there and they all do not require the same nutrients and require them at various levels. Even the people that swear to you they don't have algae, I gaurantee you they have algae if a closer examination was allowed into their water's ecosystem. There are even algae and bacteria species known to survive in ice. All ya can do is try not to disturb the algae you enjoy and do disturb the algae that becomes an eye sore.
Various bacteria species do hibernate and die off in cooler and even very warm temperatures. A change in the bacteria strength and influx of nutrients will open the doors for algae to grow even higher in population to the point where a bloom may occur. This is one reason why deep water is beneficial since the depth can provide insulation to help buffer temperature changes.
Your pond will always be cycling through out the seasons. A pond's ecosystem cycling never ends.
Always going to be a potential algae bloom around the corner. All you can do is try to control the frequency of occurences that become an eye sore.
