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Provide a roomy cage. Keep the cage clean. Feed your rats regularly. Add some variety. Avoid certain foods. Avoid using string or zip-ties to suspend baskets, toys and other items in your rat's cage. In general, the dollar store is a wonderful place to find items to decorate your rat's new home. Make sure all levels, baskets, hammocks, etc. Check that your rat cage has a solid bottom.
You will need a cage that provides at least two cubic feet of space per rat. While this is a good rule of thumb for most rats, there are some situations where rats will require more space. For instance, if you have a larger rat or multiple rats that are sensitive about sharing elbow room, such as many intact male rats, you may want a larger cage. Keep in mind that your rat is going to spend the majority of their life in their cage so giving them a spacious environment will help keep them happier when you are not able to take them out for play. You should be cleaning your rat’s cage at least once a day. For your daily routine, scoop out any visible excrement and wipe down plastic or metal parts of the cage with a baby wipe or wet paper towel. Be sure to use non-toxic, pet-safe cleaning products or regular dish soap.  Use rat litter or fleece blanket to line the bottom of the cage. Completely empty out and replace the litter at least once a week. Woodchips and dusty litter should be avoided due to the sensitivity of your rat's respiratory system especially pine, as it is toxic . Litter made from old newspaper is most ideal (ref: veterinarians), while clay litter is high in dust (despite '99.9% dust-free' labels). You can check the dust content by placing the litter in a plastic sandwich bag, shaking the bag and observing how much dust sticks to the inside of the bag. Refrain from using towels and similar fabrics in your rat's cage since your rats may end up swallowing string fibres from these fabrics, which can be harmful. Fleece is a wonderful cage liner, is reusable and is found in most dollar stores. Cloth bedding (i.e not sleeping bags or hammocks) should be changed at least once a week. If you can smell pee or other 'rat smells' in their cage or when you enter the room, then it is time to change the bedding. Again, it's well known that rats have very weak respiratory systems, so keeping your pets around smelly bedding or litter (smelly means bacteria!) can be very harmful. You should do a thorough cleaning at least once a month that includes wiping down all metal and plastic parts of the cage (bars, litter box, cage base, etc.), washing out the water bottles with soap and water, and washing any sleeping bags, hammocks, etc. Empty water bottles and refill with fresh water at least every other day. Remove any fresh food (fruits, veggies, etc.) that is not eaten. It will rot and attract bacteria. Also remove any food pellets or dried foods that have become wet. Water should be readily available at all times as rats can easily dehydrate. Have two water bottles present at all times so food and water dominance issues do not arise. Likewise, when feeding your rats snacks (veggies, seeds, fruits, etc.), it is a good idea to make two separate but equal piles. Do not give one rat something that you do not give the other rat. The rat that is missing out on the snack will smell it on the other rat and aggression can arise. The common staple food for rats is a standard lab block or lab pellets. These pellets are designed specifically to meet the dietary requirements of rats and can be supplemented with nuts, fruits, veggies, oats and other fun snacks. Oxbow rat food is a wonderful staple pellet. Do not feed your rats rabbit, hamster or other small animal pellets as the nutritional requirements of each small animal are different. It can be very difficult to meet the nutritional requirements of your rat by creating your own diet for them and missing nutrients can be detrimental to your rat's health. For this reason, it is often better to use a store-bought pellet as the staple of your rat's diet and supplement their diet with other foods. Avoid the colourful pellets at the pet store - they may seem tastier but are not often as wholesome as Oxbow or similar, and the artificial dyes can cause stomach upset. Start by feeding your rat one table spoon of these foods twice per day. Once it gets used to food being readily available, it will learn that it doesn’t need to overeat and you can leave a small bowl of food in there at all times. Rats can eat a wide variety of meats, fruits and vegetables. Try to mix in a few more wholesome foods along with the typical rat food you buy at the pet store. You can add in these foods as often as you’d like and should include roughage vegetables at least once a week.. Do not feed your rat too much protein as the pellets already have enough protein in them. Meats should be limited to one small serving once a week or very small servings (e.g. one dog kibble) a couple of times per week.  Strawberries, grapes, apples, blueberries, leafy greens, squash, cucumber, potatoes and sprouts are all excellent options for adding some variety to your rat’s diet. Once your rat is old enough to self-regulate their food intake (i.e. not over-eat), you can give your rats fresh veggies, oats and other snacks once a day. Remember to give them a nice variety so they do not get bored! For example, a yummy daily rat salad may consist of 1 bok choy leaf, 2 dandelion leaves, 1 collard green leaf (depending on size), kale and half a slice of apple or a couple of blueberries. Get to know your rat; some rats may eat everything that is in front of them while other may stop eating when they are full. You do not want your rats to fill up on veggies and not eat their pellets but if they are doing a good job of self-regulating, you can give them more than a couple of leaves and remove any uneaten leaves the next day. Meats like chicken, beef, tuna and liver are also great for rats. Make sure you mash them up a bit before serving so they’re easier to digest. There are a lot of great resources on rat diets online; be sure to read through a few before deciding on what you will allow your rat to have as you may notice some contradictions in the do's and don't's of feeding rats. For example, mangoes are toxic to male rats but can be fed to female rats. Green beans are healthy for rats but only if they are cooked; do not feed your rat raw green beans. Likewise, do not feed your rat raw brussel sprouts. Despite the stereotype, rats should not eat cheese as most rats are lactose intolerant. You should also avoid dried corn, oranges, licorice, red cabbage, rhubarb and human junk foods like potato chips and candy. All of these can be toxic for your rat. Instead, check out your local dollar store for circular, metal shower curtain rings. These rings are fantastic because they are sturdy and don't pose a choking hazard! If you are finding that hammocks and sleeping bags are on the pricey side, try using the shower curtain rings to suspend a plastic basket from the inside wall of the cage. Fill the basket with fleece pet blankets and your rats will have a very comfy nest! Levels for rat cages can also be quite pricey; instead use small, plastic organizers, dog toy ropes, pet blankets, log houses, etc., all of which can usually be found at the dollar store! **Keep in mind though that your rats will chew everything! are very secure. Rats will chew through fabric ties and anything you use to suspend or secure levels, etc. could give way at any time! Try securing levels or anything climbable in two ways (i.e. double up on the number of rings holding the basket to the cage or secure the basket to the cage wall and another object so that if one restraint gives way, the basket will not fall to the bottom of the cage). Wire bottoms are very hard on rat feet and can cause tremendous pain. If your cage has a wire bottom, line it with something smooth and solid such as plastic placemats or cardboard. Place a fleece blanket over top. If you are using cardboard, you should dispose of the cardboard each week. If you are using placemats or something similar, you should wash these each week (rats won't pee in their nest/bed but they will pee on the floor of their house; it's important to always clean up pee).