Sea fans don't really mind the horses and kind of getting use to each other. Reason for sea fans is to help absorb mysis juice and tube worms to filter out the tiny mysis particles when the horses puff. However, this mere food is not sufficient, so DT come into play.
2 to 3 cubes of mysis daily put a great burden to the sewage plant. Rowa should be used and change 1/3 bottle monthly. Charcoal 1/3 bottle montly.
Have fun and keeping a horses only tank may not really be a chore unless productive couple.....