Sorry to hear the bad news. Brook kills very quickly, it is already a miracle that Clarkii has survived this long. You have done alot to help her recover, and hopefully you have learned alot through this ordeal so her death was not wasted.
In it important to note that you should not put another fish into the main tank. You need to let the tank run fallow (fishless) for at least 4 weeks so the parasitic ciliated protozoan will die off.
In the future, make sure you quarantine all fish before you put them into the main tank. Of course, I am sure you have learned the importance of quarantine in the hard ways.