New fish always hide in my pond, for about a week, usually. Glad yours are thriving.
Do think about getting a plastic milk crate as mentioned above, or a strawberry jar type terra cotta planter, to put in the bottom of the pond for the fish to hide in. Floating or submurged plants are good, too.
Did you say you have paint on the bottom of the pond? Could this be introducing a toxin? And is there wood in the pond, too? Can you explain better? Can you post a photo?