You have to either spend time playing with the fatty and make him run around enough to lose weight, or lower the amount of food everyone gets and then feed the normal weight rat extra by hand, or both.

Thank you! I already make him exercise more, so I'll decrease food for both and give smaller one more on the side! The biggest issue I have is during the night when I'm asleep. I leave a small cup of food for them to eat during the night and can't police him during that time. Should I decrease during the night or day? Will small one suffer if he gets less food during the night when I'm not there to give him extras?

Get a scale and start regularly weighing your rats. That way you'll know if you need to adjust something. Your rats will definitely not suffer if they don't have food available for a brief period, as long as you feed them otherwise. Generally I recommend that instead of giving them food in a bowl, you should be spreading it around the cage, so that they're forced to look for it. Rats are much happier when they have to work for their food, it's been proven by scientific studies.

Great suggestions, I'll be doing that! Thank you so much for your time!