New owner

Question
I recently bought two female rats from the pet store. I have so many questions on how to properly feed them and make sure they are healthy. I've read many things on what rats like but nothing really on how much you give them how many times a week, what is good and what is bad ]: I know to check their fur, eyes, and ears, but what else can I look for. I have a one level 40 gal critter cage for them, two wheels, a log, and igloo. I feel bad because I'm so tight on $$ right now and I want my girls to have fun. I'm also worried they don't like their home or food. I change their water twice a day and their food every two days, its nutriphase. I occasionally give them carrots and bananas. I let them roam free around my room for about 30 minutes. I want to be a decent owner. Is there anything I'm missing or could I be an over-worried owner ]:

Answer
You have come to the right place! CongRATS on the new rats, btw...

LEts start from the top:

Your dong right by wanting to learn all you can so you can give the rats the best life. Good job!
You have a pair of rats which is good since rats need to be in at least a life with one cagemate.

For now the tank is ok since I assume that they are babies and if the bar spacing is too far apart they will escape a cage. However, aquariums are very bad for rats. I hate to tell you this because you want to do the right thing, but due to the fact they are prone ti respiratory infection, they do poorly in tanks because of poor air circulation and the ammonia in the rats  urine causes irritation to their lungs. Its best you get them out of the tank with the next month or two.
Also, the litter should be aspen or carefresh but never ever pine, cedar, corn cob or cat litter and saw dust. I tell why on my website.

Proper diet is not the pet store rat seed mixes and in fact, they are just too high in protein which is bad on the rats kidneys and high in fats too.  Here is the URL to my site on proper diet as well:

http://www.freewebs.com/crittercity/ratsandahealthydiet.htm

Bascially the best thing is to read over my site on proper care. It is all accurate information so there is no confusion as to what is right and what is wrong.  

Let me know when your done with it and if you need more info please let me know!  Good luck!