It’s possible that the bedding being used just isn’t effective at keeping odors down or it’s not being changed out often enough. Ideally, you want to change out bedding once to twice a week. A lot of people like CareFresh bedding, which is made from recycled paper. In my experience, it does absorb well and doesn’t have much of a smell on its own. However, I also find it expensive for what it is and too dusty for my rats. My bedding of choice is newspaper and I aim to change it out every week.
NEVER use pine and cedar in a rats cage. It’s toxic to rats and will cause internal damage in the long run.
A rats’ cage should be well ventilated, kept out of direct sunlight, and kept cleaned. Aquarium tanks are horrible as rat tanks. It traps all the odors in and the air circulation is limited. For this reason, you should try to get a wire cage like the ones shown at The Rats’ Store. When you clean the cage, make sure that you clean and disinfect the cage. Make sure you also clean the wires as well as the base.
If you have an older cage that has a plastic base or unpainted galvanized wire, it can absorb smells where even washing won’t take the odors out. Try soaking the cage in bleach (making sure you rinse it out well afterward).
Male rats smell a bit more than female since they are constantly urine marking their territory. Getting them neutered could cut down on the marking behavior and therefore, the smell.
Another reason why your rat smells bad is if they’re getting sick. Are they unkempt and is their fur out of place? This is a key sign to illness. If you have tried cleaning everything regularly for a few weeks and you still find that your rats smell, you may want to get your rat checked by a veterinarian to be sure that everything is okay.
In the end, assuming your rat is in good health, keeping their environment clean will help cut down odors. Rats are only as clean as the cage they live in. They’re meticulous animals but if their environment is unclean, they will be a little less clean.