What we really needed was the tank dimentions, and yes, 24 inches is tall enough to keep most horses. A 35 gallon hex with your stated height is big enough to keep a pair of large horses, or 3 to 4 medium (H reidi, H erectus) sized horses.
As my lovely friend Rykna already stated, a biowheel filter is not enough by itself to maintain a healthy horse tank. Besides...if your tank is 2 feet tall, I doubt an Emperor 400 will fit anywhere on the tank! (I have one sitting in the garage myself). Consider the Aquaclear that Rykna suggested, and get the larges-rated one that will hang on your tank. I have a Cascade on my 35 gallon, personally, and it's a good one too.
One thing I can't stress enough: LIVE ROCK! The filter alone just isn't going to cut it for Nitrite and Nitrate filtration. If you can, get 1 lb of live rock per gallon, so for you, try to find 40 lbs. You can always go for less actual live rock and substitute tufa rock for the remainder -- the tufa will grow bacteria colonies, and coralline algae, eventually to fill out your filtration.
You can also use a powerhead to increase some circulation in the tank, but if you do this, use. a. sponge. filter. Don't let your horse get trapped from the powerhead intake!
Last but not least......Welcome to the Hobby!!! Seahorse keeping is almost a completely separate entity from regular saltwater keeping. Don't be shy, ask lotsa questions.....there are plenty of people here who can help.