31"x15"? Is that length and height? What's the depth of the cage?
I know lots about rats. I have one boy currently because it's so hard to find rats to adopt where I am. I had a pair of young boys but unfortunately lost one, he just wasn't thriving at all. So now my Duquesne is all alone.
Mazuri isn't good for rats, neither is Oxbow. The best rat block is Harlan Teklad, now called Native Earth I think. You can feed a Suebee's type mix along with that and fresh foods. Or you can do a high quality, low protein dog food with a Suebee's mix and fresh foods. I don't often give baby food as it's not all that great for them. A little fresh meat like chicken, turkey or tuna is good, hardboiled eggs occasionally, small amounts of fresh veggies, yogurt, organic or whole grained pasta's both cooked and uncooked, a little piece of fruit occasionally, a mini m&m a couple times a week as a special treat, hard-shelled nuts once in a while.
I cook up stuff for them like grains such as amaranth, quinoa, brown or wild rice, oatmeal, whole grain pasta and then I will add in a small amount of turkey, chicken or tuna. I do this a couple times a week. I give fresh veggies daily but only a couple things. One day it might be a piece of bell pepper and a piece of cucumber and the next it could be a small piece of lettuce and a couple sprigs of cilantro. I also give thawed frozen peas occasionally.
I make up a Suebee's type mix (ingredients vary sometimes from each batch) but I start with a box of total cereal (it contains all the needed vitamins/minerals) then add in puffed cereals such as puffed wheat, puffed brown rice, puffed millet, puffed corn, puffed kamut and kashi 7 grain puffs. I also add in some type of Nature's Path granola cereal with fruit and then 2-3 other good grain flaked cereals such as Nature's Path Heritage Flakes (and another Nature's Path cereal that has amaranth in it but can't think of the name and don't have the box as I ran out of it). I have gone to the better cereals for my rats as most of the stuff you get in the grocery store is too sugar laden. I use Nature's Path, Arrowhead Mills and Kashi cereals for the majority of the mix. I do use wheat chex and multi grain cheerios also but I only put about 1/4 box per batch I make. I also add in a small amount of rolled oats, unsalted roasted soynuts, 1 small package of a tropical dried fruit mix. I'm sure I am forgetting some stuff I put in the mix but it gives them a huge variety of good things to eat. I make a large batch and it lasts quite a while with one or two rats. Duquesne picks out his favorites first but he eats everything I give him and so have all the other rats I have had. I don't give a lot of the cereal mix. Usually one small dish full each night (the small colorful dish like you find for hamsters at Wal-Mart). It would probably equate out to about 1/4 cup.
Along with the mix, I give a high quality Senior dog food that is low in protein. I use Innova Sr dry. I give a few pieces each day along with the grain mix.
Treats- those baby puffs you can get in the baby aisle make great treats as they are very low in sugars. There are several flavors.
Like with pigs, you want to stay away from store bought pet treats like yogurt drops. The baby yogurt melts are much better as is fresh yogurt. They also cannot digest alfalfa so nothing with alfalfa in it like those huts or logs that are out for rodents.
