Rats should NEVER be kept in tanks. They do not have enough ventalation. Rats have very sensitive respitory systems and the ammonia will hurt them. That is also the reason that softwoods can't be used as bedding (ex. Pine or ceder shavings). The phenol oils in them are harmful to them.
Female rats tend to be much more active then the males and often in petstores (I've worked in Super Pet and did a lot of educating to the staff about rat care) they have them cramped together in to small a cage with not enough room. If you have enough cage space any number of rats can be housed together, they should have AT MINIMUN 2 cubic feet for a pair of rats. I have 6 wonderful girls housed together and they love it. The cage I have them in is also quite big and I'm saving up to get 2 Ferret Nation (external - login to view)
cages to attach together for my girls. I also have 2 boys and as they get larger then females they need more space, I advise 2.5 cubic feet for a pair of boys at minimum. This (external - login to view)
is a cage with the same mesurments as the one I have for my boys. Technically it would be ok for up to 3 males but the more room the better is the rule with rat cages.
Also rats ABSOLUTLY MUST
be in pairs at minimum! It is NOT healthy for rats to be alone. They either need a same-gender companion, or if you have one of each gender one or both need to be sayed and/or nutered. It's recomended that you get all female rats spayed as they are prone to cancer and this cuts down the risk greatly.
Rats do make fantastic pets. And anyone who is willing to do the research and put the care and money into them should think about it. But people should ALWAYS do research before getting a pair, rats require some special care and not learning about them could do some serious harm.