Well I agree with Lois that this tank is awfully young, but as the OP mentioned, the horses came sooner than expected. Sometimes, S**t happens. So as she says....it's definitely all hands on deck for damage control mode.
Nikki, fist of all you should definitely continue with water changes.....how much are you changing? Since you have ammonia, I would recommend you go with water changes of at least 15 liters, every other day is probably OK. Try to change out 30% of the tank water each time, and you will probably want to add some PRIME or similar ammonia-reducing chemical.
For your final tank, 3 horses are going to need a tank in the 130L range. Try to find one that is 60 to 70cm tall, since horses like to use this vertical space. 60cm should be fine; anything taller is just bonus.
I also noticed you're setting your tank to 25C. Please drop this to 23...it's much more comfortable and healthy for the horses. I don't know where you live, but you may need a chiller to keep this temperature stable. The lower temperature will also help the horses' overall health considering the less-than-perfect chemistry they're dealing with right now.