Diet is the most important aspect regarding your rat's health and length of life! Rats need to eat a food that has been specially formulated to be nutritionally complete for them.
Fun fact, Rats are opportunistic omnivores! This means that they eat just about anything they can find. They will eat things from grains, veggies and berries to bugs, fish and smaller mammals. Because of this, a lot of owners believe that their rats will be okay eating scraps and other random foods/random mixes. Sure, they will survive off it for a while, because that's their nature, but they most definitely will be unhealthy and have shortened lifespans.
There are so many foods out there marketed for rats, but very few are nutritionally complete. Random "rat foods" from stores are not a complete diet for your rats. It is very important to make sure your rats nutritional needs are being met. The best, and nutritionally balanced foods for rats are; - Oxbow Essentials Adult Rat Food - Science Selective Rat Food - Mazuri Rat and Mouse Diet - Harlan-Teklad Spunky Rats recommends Oxbow Essentials Adult or Science Selective.
When feeding other foods that aren't nutritionally complete, your rats will be lacking nutritionally which will lead to long term health issues and shorter lifespans. A lot of people think of it as "they're just rats, look at what they eat in the wild.", but wild rats very rarely live past 6-12 moths of age, where pet rats can live for 3-4 years(a healthy diet TRIPLES their lifespan!). This is mostly because pet rats have access to everything that they need in their diet in one simple block of food. Of course, there are other factors to shorter lifespans in wild rats compared to pet rats, but I'm not going to get into it here.
Feeding a mix When feeding a mix, you must follow a nutritionally complete recipe or purchase a nutritionally complete mix. Do not feed store bought mixes, they are nowhere close to being a complete diet and most contain unsafe ingredients (whole dried corn being one). Food mixes are a lot of work, they have to be carefully crafted and fed to be balanced. You will need to do a lot of math, measuring and planning to create a balanced mix. When feeding a mix, the food will have to be scatter fed so that its harder for the rats to pick and choose the things they like out of it. Although it can make it difficult for them to pick and choose, it is still possible, and it can cause the rats to lack in certain areas of their diet. This can easily lead to deficiencies and can cause short-term and even long-term health issues. If you're still wanting to feed a mix knowing the risks, Spunky Rats recommends Purchasing a mix, or recipe from Igloo Rats.
Seed mixes as treats? Spunky Rats only recommends seed mixes that are made from home, with ingredients purchased from the grocery store. Seed mixes from the pet store made for rats or birds are not a healthy treat to feed your rats. The ingredients aren't food grade, for instance you wouldn't go buy a bag of bird food for your dinner.
Fresh Foods are also important! They should make up 20% of your rats' diet, so fed 2-3 times a week in small amounts. These make great, healthy treats for your rats and also provide enrichment if used that way! Vegetables, Fruits, Meats(unseasoned), and insects are most common. Here's a short list of safe foods; -Peas -Corn -Kale -Carrots -Tomatoes -Spring mix greens -Green/yellow beans -Watermelon -Strawberries -Cherries -Apples -Peaches -Bananas -Chicken -Turkey -Eggs (boiled or scrambled) -Captive bred live or dried mealworms -Captive bred live or dried crickets -Captive bred live or dried grasshoppers -Captive bred live or dried shrimp