Watch out for lab blocks because they are such a commercial wide spectrum food they often contain too much protein for rats, which eat a more grain based diet. try a home amde diet if you are close to a bulk food store, its cheap and simple and lasts much longer then store bought foods. blocks are also boring
these sites contain good rat food options
http://www.ratsrule.com/diet.html http://www.geocities.com/ratsonthehill/diet.html