It will work, but here is a better question. Is the flow output important for you on the Magnum??
This reactor will impede the flow quite a bit. It actually would do a better job of dissolving the CO2 at the cost of the weaker flow. Good thing if Co2 dissolution is important to you, bad if you want flow.
I used this on an Eheim 2028 and I get bubble spewing out from the output like mad cause the pump of the 2028 is too dang strong. I get 100% dissolution when used on 2213. Great, but of course flow is quartered at least.
Other than that as long as the tubing fit then it's good to go.