You were right about the degrading buffering ability of vinegar-- I managed to bring it down to 8.0 before the fish came, but when I retested yesterday, it had bounced back up to 8.3 and I am out of vinegar. Will add food grade Phosphoric acid to the shopping list.
Truth on adding biology! Since the fish are tiny (1-1.5"), they aren't keeping up with the vigor of the duckweed, which I guess is a start. I read that taro is water loving and picked up a stalk at the grocery store and dropped it in the watering can just to see what would happen... Nothing so far. A "super dwarf cavendish musa" (Musa acuminata) is also on its way. They say the banana tree should stay sub-4 feet. It may not fit in the watering pot, but it can probably soak its roots in a similarly diametered planter, at least until the arrangement becomes too precarious. I wonder if when its canopy comes in it might help stem some of the evaporation off the pond? In semi-arid socal, it seems the pond level can drop as much as an inch in a day. How do other folks combat evaporation? There would be less surface area on a 55 gallon plastic barrel but then would I need to worry more about aeration and limit the plant options?
Didn't realize adding ammonia might actually be beneficial-- I have an abundance of worm castings and can probably macgyver a strainer bag equivalent in a couple mins
Answers to your q's--
How big are these new fish?
--fingerlings1-1.5". There were 14 originally, it is tough to get them to hold still long enough to count past 10 at the minute. They don't seem too densely stocked for their current size.
Do you have any kind of filter system?
--not at the minute, potentially in a future state. We found some rad videos of folks running hydroponic sluices-- PVC tubes they had hole sawed grow baskets into. They had to pump the water to the top, but then it could gravity feed its way through 90 feet of PVC from there. A little worried about fully integrating this sort of filtration with the fish pond until we know what we're doing. We may try changing out some of their water and feeding the dirty water through this sort of setup to see how it goes. Is there a more elegant way to filter? The pond already feels a bit like it is on life support with its air pump and heater plugged in.
Is there an air pump or are the bubbles under the watering can coming from the water draining back down into the tank?
--that bubbling is an air pump we anchored under an overturned clay pot that the watering can is sitting on. We didn't have aquarium-specific tubing or air stones, so it is an old drip irrigation hose free flowing.''
Thanks again for your help!