If the pump has no speed adjustment you can try redirecting the nozzle so the heavy current goes into the nearest wall. If its a hang on back keep the water level up and if you can't get some decorations right under the outflow to absorb some of the current.
In either case, as long as the tank isn't tiny, you fish should build up the strentgh for the current over time. As long as he can escape someplace out of the current I wouldn't see it as a problem.