Hey Lars,
Overall, Cupramine is incredibly safe to use. Just make sure at a min you follow their instructions to spread the dose out. I do check it with a test kit. For those fish that don't react well with it- I really take my time to ramp up the dosage. I keep a diary for all tanks.
With the Prazi- I follow their directions. After the first round I do a wc but do not always run carbon and redose. I tend to dose the Prazi 3 times.
I tend to does the Prazi first a couple of rounds and then the Cupramine. I do not use them concurrently.
For filtration- the 250 has a 75 gallon sump/fuge and 2 Eheim Pro 3 1200XL canisters....the 125 does not have a sump/fuge but also has 2 Eheim Pro 3 1200XL canisters.