After doing some research, I've come up with lots of information.
The smallest snakehead available still grows to 8 inches long. It is the Dwarf Snakehead. Even they alone require 75 gallons because 1) the get BIG 2) they are FAST and 3)They are AGGRESSIVE.
Anything other than this species will need 100+ gallons, and more than one. When alone, they become timid and feel insecure. They will hide most of the time, and be extremely boring for you.
I also read that when you get your larger tank for it (I strongly recommend getting 150 gallons or larger to be safe), that the glass needs to be reinforced in case the snakehead decides to
ram the glass.
Also- They should be in a species-only tank. They will eat anything and everything you put with them. He may not be aggressive now, but he will be as he gets bigger, faster, and stronger.
Finally, the Snakehead is an invasive species in the USA. I'm not sure where you are from, but for most states, it is illegal to keep them. You should look into whether or not you are okay to keep them! Where I'm from, they are not allowed.
Have fun!