Hi stricknine,
Sorry your reidi is having problems. As Ccall mentioned, we need much more information in order to help.
Please post your current water perams in actual numbers, Ammonia,Nitrite,Nitrate,PH and temp.
We need to know as much info as you can provide, tank setup,tank mates, eating, respiration of the horse, any physical abnormalities that you can see, such as cuts,scrapes,open wounds, swelling,etc.
Has the horse stopped eating, if so, how long has it been since she last ate?