Rather than using strong store bought chemicals you can try making a DIY leave in conditioner for your horses tail that is all natural. I've made one with all different kinds of oils. Also be extra careful when brushing your horses tail!
Tail bag if that's possible, also just try bruthing it with a hard brush rather than a main and tail brush so that you're not pulling out the hairs and untangle knots with your fingers. Also if you're showing put in a false tail