solid effort on the behalf of the bristlenose! to survive with turtles is a tough ask for any fish.
Like the others my advice would be to take him out and give him his own tank. I do not think that he is in too much danger as turtles are pelagic hunters (they eat things in the water column, not on the bottom) but they might give him a nip while he is on the sides.
Other than that, he will be happy eating the slime and algae in the tank and any leftover food from the turtles. They are tough little dudes.
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