Well, I agree with the above comments. But since you already got one and you're building a big enclosure for it, then I might as well add my few cents in the proper care of these animals.
First of all, niles aren't the smartest or social of reptiles. So you must be able to cope with a pretty anti-social animal until you're able to "tame" it. Now, how do you "tame" a nile? Well, most people just try to forcibly handle the lizard and expect it to get use to it. It doesn't work that way. The way I go about it is offering food using a tong, dangling it in front of the animal and let it come to me. The same way your fish would come to you because they associate you with food. After a few weeks, the animal should be comfortable enough that it will approach you. Then, I would extend one arm into the cage with my hand resting on the bottom, and dangle the food with my other hand. The lizard should climb onto the arm in order to get to the food, once it's on my arm I slowly move it out of the cage. The whole idea is to let the lizard come to you, rather than you running after it. However, I've seriously not heard of a dog tame nile. They get semi-calm at best, so you basically have a big, carnivorous temper prone lizard that will (at best) tolerate you.
There's a reason why they're one of the cheapest monitors around. These just aren't really considered pet quality animals.
You should dust it's food in vitamins and calcium powder once a week. Being scavengers, they'll eat anything. So they have no specific or delicate dietary needs.
The adult enclosure should be big, about the size of a small room. Since these are terrestrial animals, and get very big, a good 15 ft by 10 ft enclosure should be provided.