Hold on, I think that I detect a misunderstanding here.
Just because a PSU is rated at 25A, does not mean that it will always supply 25A to the load. The load will take what it wants, you dont have to "dissipate" 18A (I assume to make the total load 25A).
What current does your plating bath take when connected to the PSU?
7amps?
If 7A is what you need, job done, you dont need to waste the other 18amps to warm up the room.
However, if your plating bath takes more than 7A, you need a resistor in series with the supply, not a resistor in parallel to "split the current".
JimB