If the filter doesn't come with a "speed control" kind of thing, I understand that the only way would be to change/reduce the power supply of the filter - perhaps you can find someone who can tamper with the electrics of this
What I have done with my external filter is to use a special output tube...this tube has several small holes that hence disperses outflowing water to a larger surface than just one strong outflow. I found this easily from my lsf I think.
Have a think on how the water is flowing in your tank. Is there a way to place large ornaments that can block the flow (not completely) such as to create a peaceful area for the fish? I also have a large hollow ornament that protects the goldies from the strong part of the current - my baby goldies seem to have learnt to utilise this space after a while