General rule for tanks is 20 gallons for 1, and 10 gallons for each rat after that. Since your using a topper I would say 15 gallons for one, 5 or 10 gallons for each one after that.
20L Aquarium Tank Topper Cage Funhouse Gerbil Rat ..
Tanks are especially bad for rats. They really do tend to have poor ventilation. Not to mention you would probably need around 100 gallons (73 x 19 x 20) for two rats, which would cost at least $250, when you can get a much better cage for a lot cheaper. And of course, you would have to add a tank topper both to increase the ventilation and to add extra levels (which rats need), but I don't even know if there are 100 gallon tank toppers available. And if there were, they would be incredibly expensive.
How to make a Rat Tank Topper.? | Yahoo Answers
The Tank Topper fits virtually all standard 10 gallon aquariums. Compare your tank's measurements with the Tank Topper before buying to ensure a good fit. The Tank Topper can be expanded to include additional CritterTrail accessories (sold separately).
tanks are never suitable for rats, even with a topper.
As far as the tank, considering she is hairless a large enough tank would be just fine for one rat. I usually do not recommend tanks, but with a large tank cleaned fairly often it should cause no trouble at all. Her having no fur and no furred company does make me a little concerned she may be getting cold, which is why I think the tank may be best until you get her a friend. Or, you can add lots of baby blankets or other snuggly items so she can warm herself. I know my doe Wrinkles can get cold fairly quickly, which is why she is constantly looking for clothes to steal and bring into her cage. If she has none, she actually bundles up her bedding in her mouth and brings it to the top shelf so she can have something to keep her warm.I've recently acquired a 20 gallon tank that would be a good option for my ratties... as long as I can find an aquarium topper! These are the cages that go on top of the aquarium so they get multiple levels and better air circulation. Does anyone, in Canada, know where I can find one? I called all the big and small pet stores around me and no one seems to carry them. One store did but only in the 10 gallon size.As far as tank toppers are concerned, a ten gallon with a tank topper is much too small for one rat, let alone two. I do not recommened tank toppers at all, unless they are powder coated. They tend to be very dirty, having the urine leak down the tank and it's a constant clean up. I use my tank topper for recently weaned babies who are unable to go into the bigger cages until they have grown some and are into their new homes, which isn't long. They do however, make larger tank toppers. They are fairly easy to make homemade and if you need help making one let me know. :)tanks (even with toppers) are poorly ventilated, and are very likely to cause respiratory issues in rats. frequent respiratory infections can weaken them, and damage their respiratory tract. as well as costing a lot to treat recurrent illnesses.