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!)