How to stop fighting rats?

So i asked before how to help a mourning rat because i had a rat that passed away and his cage mate became depressed , they all said to get a new rat. So i bought a new rat who is four months old and about half Alejandro's size. I started introducing them slowly, but then when i put them in the same cage Alejandro started attacking him, he actually drew blood. So now they are in separate cages, but i only had a drawer type thing to put the new rat in. it's a lot like a fish tank, but i don't want to keep him there, so is there any way to stop them from fighting? Because otherwise I'm going to have to give him back to the pet Store. Please help!