Whatever you do, IF you decide to treat the horses with an antibiotic, please, FINISH THE ENTIRE TREATMENT as directed: correct amount per dose, correct schedule of water changes, correct number of doses per day, correct number of days. If you give them a partial treatment, you will be killing off the weakest bacterial organisms and allowing the tougher, more resistant ones to reproduce, thus insuring that any other animals you introduce to your system will be subject to a stronger, more treatment-resistant strain of bacteria.