Conditions
You don't have to be exact, but your rats should should be in temperatues between 64 and 79 degrees Farenheit. Your cage should not be in direct sunlight.
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Housing Size
Sources range from saying 4 square feet for two rats (6 for three) to 200 square inches is adequate for floorspace for your rats, but regardless of the minimum more is always better and there are great cages with 800 square inches of floorspace in the rat supplies tab. Rats like to climb so the cage should have multiple levels and each level should be tall enough for your rats to stand up in. It is reccommended for the cage to be at least 50 inches tall so your rat can climb. Rats also need space to burrow. If your cage, which would be wired, does not have a bin for bedding or not a deep enough one, you can fix this - how is mentioned below. The bars of your cage should be 1/2 inches apart. Male and larger female rats will not be able to get through 1 inch spaced bars but in case you have a smaller rat, and just to be safer, 1/2 spaced bars is best.
Cage Types
The best cage for rats is a large and tall wired cage. The wires allow for your rats to climb, but since this means they can bite the wires, which will damage their teeth, it is extra important to have a larger cage regardless of the reeccommended minimum. Biting on wires is a boredom behavior and can be prevented by a big enough cage with enough enrichment. Oftentimes wired cages have a too small bin for bedding, and the wired cages big enough for rats may not veen have a bin for bedding at all. You can line the bottom of the cage with cardboard or another material, like chloroplast. Your rats could bite through cardboard so if you choose that option you will likely need to replace it every so often. Despite this it may be a good option as many people already have a lot of cardboard and not many people have chloroplast laying around so you would need to buy it. You would need to measure each side of the cage and cut pieces to that and put them to the sides of the cage. The pieces about 8 or more inches tall. Some of the cage can have less bedding, like 5 inches, but at ;east some part should have 8 or more so your rats have ample burrowing room. Not only do they need enough room for them selves,but also, multiple rats will be burrowing so they need space from each other.
Bedding
As mentioned above, rats should have at least one section of their cage with enough bedding, which should be 8 or more inches of bedding. The bedding should be dust free and safe, meaning unscented, and not made of wood shavings, saw dust, or cedar. Paper bedding is safe, but pine allows for more ammonia control. To not disturb burrows and not stress out your rats by removing what smells like them (their old bedding), spot clean where they go to the bathroom and clean out sections of bedding every so often - never all of it at once.
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Sand Bath
To remove excess oils and all around clean themselves, you can give your rats a sand bath about once a week. You can get a bin to put the sand in and let your rats in the bin once a week. Safe sand would have no chemicals and no scent. There should be little to no dust and the sand should not be too fine to avoid respitory issues.
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Cage Decoration
To keep your rats entertained when they are in their cage, there needs to be a lot of things in their cage to enrich and entertain them.
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Food and Water Containers
You should have a food bowl - two is okay too so the bowl does not get crowded with all the rats, but put less in each bowl if you do too, because you should also scatter some food around the cage to encourage natural foraging instincts and enrich your rats. You should have 2-3 water sources. If you only have two rats, you are fine with 2 sources. You can have bottles or cups of water. The cup should be about 2 by 1 inches. It may be a good idea to have one of each in case the bottle gets an air bubble. This may happen often and can be easy to fix by you tapping the ball in the straw but with a cup your rats will have another water source before you fix the bottle (two bottles would also prevent this but this is safer because both bottles might get an air pocket before you fix any of them).
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Wheel
So your rats can run to exert energy and entertain themselves, rats need wheels. To not hurt their backs, the wheel should be 12 or more inches in diameter. It can never be too big so if you can, you can go above 12 inches, though you would have to go online because typically stores do not have above 12 inch wheels. It would be best to have a wheel per rat so they can all run when they want to. If you have more than one rat two wheels may be okay because not every rat may want to be running but there should be at least two.
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Hide Houses
As animals of prey, your rats need a lot of spaces to hide when they want to feel safer and more comfortable. You should have quite a lot of hides, but at least 2-3 per rat. You can get hammocks or hides that hang from the cage and you can get hides that attach to the side bars of the cage. You can also get hides that will sit on a platform. Since you need quite a few hides, to save money, you can make one or two hides that will sit on a base out of cardboard. You should have a variety of textures and sizes of hides so your rats can choose what they like and be enriched by the variety. You should avoid plastic because it does not properly block light and your rats may chew on the edges of their hides, and you do not want them accidentally consuming plastic.
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Sprays and Hay
You can put some sprays like flax and millet sprays in your rats' cage for nutrition. They will be enriched when they have to pick the seeds off of the sprays. Rats do not eat hay but it can be used to strengthen their burrows and give them new textures in their cage.
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Chew Toys
Rats' teeth are always growing and they grind their teeth to keep them down, but chew toys can help with this, along with enriching your rats when they chew. You should have a high variety of chews for your rats. Different textures and flavors is more entertaining than it all being the same. You should also have many chews so there is enough for each rat. As goes with everything, the more rats you have, the more you need. To further enrich your rats, you can get puzzle chews which invlolve pieces for your rat to move around, and usually involve finding food.
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Other Enrichment Objects
You can put rocks in your enclosure so it will shave down your rats' nails as they walk on them. You can add objects like cork and grapevine logs and empty toilet paper rolls in the cage for more things for your rats to walk on and around and nibble on. You can shred toilet paper and put it around the cage for your rats to take to their nest. You can use an empty tissue box as a DIY hide house.