Hi Kamil,
Forget the bishopi, they are very rare now and completely off limits for collecting. They are not much smaller anyway.
I am not trying to preach here, but in my opinion hellbenders should only be kept by those trying to breed them. I can't see the justification in someone collecting a single animal for use as a 'pet' or showpiece. They may be somewhat common locally, but overall, they are rare. In a 1 mile strech of stream ,there may be only a few feet of that stream that is suitable habitat. Alot of populations are non viable, with only large, old adults-no young.
I understand they are very interesting and appealing animals,so don't take this the wrong way.Just my opinion and since you asked, for one hellbender, you would need a tank of about 55 gallons minimum.A tank of 75-125 gallons is more suitable though, with water temps under 75F and flowing water.
I am confident that someday soon, they will be bred in captivity, so hang in there. I did have a female lay eggs in 2000, the male was very small, but mature and he didn't do his job. At least it was a step in the right direction. I have a new plan for breeding them(outside with a chiller when necessary) and I am confident it will work. My trouble now is with getting permits to keep hellbenders in the state I just moved to. Had to leave mine with a friend back home for the time being.
If you have any other questions, just let me know....