20 gallon aquariums are ideal and should be the minimum amount of space assuming you have a pair as they give your gerbils plenty of space. We don't recommend pet store plastic/wire cages as they tend to be to small and gets messy quick, as the gerbils throw the bedding out through the bars.
If you have gotten a glass aquarium you need a lid and you can almost always find a wire aquarium topper at your local pet store. If you are looking to give your gerbils more space we highly recommend a tank topper. Tank toppers have space for wheels, water bottles, food bowls and even small hideout giving your gerbil plenty of space for borrowing in the tank.
Aspen shavings and soft paper bedding are recommended materials, found in most pet stores. These materials are absorbent, soft and good for burrowing. We would not recommend Pine and cedar shavings as they are bad for small animals lungs.
You will need a wheel your gerbils can get their energy out on especially in 15 gallons aquariums. To sum it up there are two kinds of wheels you can chose from: A plastic wheel or a metal wheel. both of them have their pluses and minuses. The plastic wheel is great for younger gerbils because their claws and tails wont get stuck. But the metal wheel is usually quite large so the gerbils can't out grow it. This wheel also provides extra track and is chew proof.
Gerbils do not drink much, unlike humans, as they're originally desert animals. But it's important that they always have fresh water. A water bottle with a metal holder is essential.
Unless you have your gerbil on a special diet, the brand of food you get doesn't really matter, but personally we recommend mixing at least two different bags of quality gerbil-specific food together. Make sure you get a bowl that is chew proof, either glass, clay or metal.
Gerbils, like all other rodents, need to chew things to keep their teeth from growing to long. Luckily you don't have to buy anything to get chew toys if you don't want to. You can use lots of items from home that you usually have to recycle. For example you can give them toilet paper tubes, paper with no dye or cardboard boxes. Just make sure there is no tape or plastic on given objects. Other ways to enrich your gerbils further is with things like fresh vegetables/fruits, hamster balls or millet sprigs. When giving gerbils fresh vegetables or fruits always err on the side of cation as grapes, onions, produce with citrus and some other produce are unhealthy and can even intoxicate your gerbil.