What to Feed Your Rats

Everyday
Your rat's diet should consist of 80% lab blocks and 20% fruits and veggies.
I personally like using Harlan Teklad 2014 lab blocks because they're not that much more expensive than the common brand but a little higher in quality. This is only available online or purchasing it through rat rescues. Petfooddirect also offers Harlad Teklad under the brand name "Native Earth" and they also have an auto-ship program so that you can receive it through regular intervals at your doorstep.
You can also supplement your diet with a grain mix. There's a popular mix called "Sues Bees" mix but there isn't very much nutritional value in that mix. Puffed cereal contains very little nutritional value for the rats. Dog food should not be fed in the quantities listed on the Sues Bees. Here is a basic recipe that you can feed your rats:
5 cups of oats (any variety, flaked, rolled, whole wheat, etc) 2.5 cups of seven grain cereal
2 cups of crumbled post op shreddies (no added sugar or frosting)
1/2 cup bran flakes
1/2 cup barley
1/2 tbsp flax seed
2 tbsp sunflower seeds
2 tbsp pumpkin seeds
3 tbsp cheerios
1 cup wheat
1 cup rye
4 tbsp unsweetened shredded coconut
1 cup dried banana
1 cup other dried fruit
1/2 cup roasted soy beans
2 tbsp soy beans
1.5 cups of dried veggies
1/2 cup of dried tomato or veggie pasta
Sparingly
I like to use separate treats for when I'm training my rat to do certain tricks. I used to break up Milk-bones (yes, for dogs) and give them bits and pieces of that. However, I found that they gained a LOT of weight. Here are some healthier things you can try:

• Unsweetened cheerios
• Shredded carrots
• Milk-bones
• Sweet potato
• Romaine lettuce
• Strawberries

Water
Water should be provided all the time. You can use a bowl (which requires more frequent changing). The primary problem with bowls is that a) your rat can fall in, b) bedding, poop, food and all sorts can end up in there and c) it can spill making a huge mess.
There are also plastic water bottles available that can attach to the cage, however, some rats like to chew through it. I personally prefer buying the glass ones. It's sturdier and doesn't shift around as easily.
The 8oz water bottles will probably work for one rat but you'd be better off just getting the larger ones. I have a 32oz water bottle since it gets hot where I live and rats will drink more during the warmer days.