Are you worried that the heater will go out and cook the fish, or is the heater actually broken?
Also, what size tank is it? In larger tanks (55+), I like to use two smaller heaters rather than one large one. The same principle could work with in-line heaters I suppose: put one on the way to your sump/canister and one on the return.
Other than that, I haven't ever used a heater controller, so someone else will have some better insight.
Im worried that it will go crazy and cook the fish. I have several tanks with 100 and 200 watt heaters. The inline is going on a Fluval G6 (75 gal ). I like the idea of two smaller heaters but, didnt realize I could use two inline.