I have two goldfish in an outdoor pond that have had persistent black spots that look depressed. These spots are on their sides.  Is this black spot caused by ammonia surges as I have read? Is there any treatment that will restore the fish?  It is a 2500 gal pond with a skimmer, filter, uv light, more filters in the water fall along with a bag of stones. I have 25 fish, 20 of them large, also two water lilies, and I use a barley water treatment weekly.

Yes, they're most likely ammonia burns that are healing, so do test your water! And if you notice that there is a high level of ammonia present, you need to start testing it every day and changing it whenever it spikes. Change about half of the water at a time or something.

Hanna: When there's a waterfall in the pond, that's enough as an aerator, it works the same (Could be good to have some "emergency airstones" though for the hot days!)