Some rats are chewers - they just tend to chew more than others. I have one girl who kept gnawing on her martins bars for a while, I think I have FINALLY gotten enough different varieties of chew toys in there to get her to stop - she wasn't damaging anything but it was making me kinda crazy to see her chewing. The other girls in her cage never bothered. Its a personality thing, but generally once they discover they can chew something, they keep doing it. (She still chews the bars...and its just a couple bars in one spot...now and then, she just also chews on the other things I provided too). I would avoid any cage where plastic edges are accessible and related to the containment, since with a chewer that poses an escape risk. Do they have a wheel? Wheels are a great way for them to burn energy, and tend to be well accepted and used, especially by female rats. Make sure to get a mesh surface (not bars), or solid surface running wheel for their safety, and that might help her burn some of the energy that is driving her to chew (won't stop it though, will just reduce it).
I had 2 rats in a 40g breeder critter cage for a while YEARS ago, when my living situation offered me the option of 'solid sided housing' or 'get rid of the rats' (my landlords at the time decided their preexisting mouse problem was to be blamed on the presence of my rats 'attracting' the mice...even though it should really do the opposite...). I even felt that THAT was kinda crowded.
To prevent ammonia buildup it was necessary to clean it at LEAST every other day, and preferably daily, I had all sorts of other things inside added to create levels for them to climb, hang hammocks, hang a wheel on, etc (wheels are GREAT for helping girls burn energy). But I was VERY happy when I finally moved out and was able to go back to using a cage. I do still have to wonder if the one girl's chronic respiratory issues weren't due to having spent several months living in an aquarium, despite my extra efforts to keep it clean and well ventilated.
I also had to repair the top very regularly. They chewed it and tore it apart frequently. Generally the rule can be taken that if they figure out they CAN chew it and do damage, they WILL. (This set of girls also decided that plastic water bottles were good chew toys, I went through 3 in a week when they discovered that (after an extended time of no problems) before I found glass ones.) That's another reason a cage tends to work better, the better designed ones (martins, critter nation, etc) don't have any plastic they can accessibly chew (the sides of the martins trays are outside the bars, and therefore out of range of teeth, vs the superpet ones that sit ON the plastic tray instead of in it, making the edges available for chewers). I would strongly recommend looking into a critter nation or martins cage for them. The ventilation will be much better, it provides climbing exercise too (a wheel is still almost a must with females though!), and less of an issue with chewing potentially leading to an escape issue.
With CAGES (with 4 sided and top ventilation) it is recommended rats have a MINIMUM of 2 cubic feet per rat. I honestly believe that with the reduced air flow, aquariums require more space than that, and unfortunately a 40 gallon just does not provide that (if it were a cage of the same dimensions, it would support 3 if all the space were carefully used, which is only half of what your tank is currently housing). Not to mention tanks are heavy, which can make it difficult to move them to thoroughly clean them. I would recommend you look into either getting a powdercoated R-695 cage from martins cages ( http://www.martinscages.com/products/cages/rat/
) ...which technically still only supports 5 rats, but would be a far better setup and much less likely to lead to problems (its about twice the size of a 40g breeder tank, with better ventilation and less escape risk!), or a critter nation (a single level can support 6 rats if used well, at 36"x24"x24"), as these would provide your rats with much needed space, ventilation, and would reduce the risk of the one chewing her way out of the top eventually. With a tank...for 6 rats you'd really be looking at a MINIMUM size of 90 (48x18x24...6 rats at the 2 cubic foot rule...which I think is too small for a tank living situation)-120 gallons (48x24x24) with cleaning at least 3 times a week, preferably more. Tank toppers are also an option if you can find one big enough (martins cages could custom make one for you I'm sure), but the bottom part would still need to be cleaned several times a week at a minimum, since the ammonia can still build up there and cause problems (the cage top just lets them escape from it better, and gives them much needed climbing space). So tank topper can HELP, but aren't really an ideal solution. If your living situation absolutely requires the housing must be solid sided (in which case a tank topper probably wouldn't be allowed either), then looking for a larger tank, and finding a more secure top for it (possibly making one or being sure to purchase a top that is all metal, etc), is important for the number of rats you have there. But I think you and your rats will be happier with a cage, as far as space, expense, time needed for cleaning, health, weight of cage, security, etc etc. So please look into that as a possible option (and moving to a secure cage would stop her from chewing the top of the aquarium as well!).