Contrary to their name air plants can’t live on air alone. While air plants don't grow in soil, they do require light and water. In nature Tillandsias roots attach and grow on rocks and trees and absorb moisture and nutrients through their leaves.
Light:
Air plants prefer bright, indirect sunlight or fluorescent home/office lighting. If your plants are in a room with direct sunlight place them several feet away from the windows and mist every couple of days to keep them hydrated.
Water:
Air plants are drought tolerant, but if left too long they will not grow or thrive and will eventually die.
Once every one to two weeks (depending on humidity levels in your home) soak plants upside down in a bowl of water for 30 - 40 minutes. After their bath gently shake off any excess water. If your terrarium is such that your plants do not get enough air circulation to dry in 4 hours let them dry before placing back in their home (Air plants require good air circulation and will not survive in a closed container).
In a drier climate, such as Alberta, or where humidity levels are low an additional light mist once or twice is week is preferred or mist every few days if your plants are receiving more than two hours per day of direct sunlight.
Air plants are very forgiving so if you forget to water one week or take a much needed vacation your plants will be fine.
If your air plant is in flower be careful to water so that the flower petals do not get wet.
Sea Urchins
If you have an air plant mounted in a sea urchin you can either give them a water bath as above (shell side up and after bath time ensure there is no excess water left in shell) or mist well once per week and lightly a second time during the week (depending on humidity levels in your home).
Maintenance:
It is normal for some of the lower leaves of your tillandsias to dry out as the plant grows or acclimates to its new environment. Those leaves can be gently pulled off of the plant. If your plants leaf ends are dried just snip the dried tip off, trimming at an angle to leave a natural-looking pointy tip.
Fertilizer:
Fertilizing your plants is not necessary, but doing so monthly will keep them in top shape and promote blooming. You can purchase air plant fertilizer or use any water-soluble fertilizers at a 1/4 strength (Rapid Grow, Miracle-Grow, houseplant fertilizer, etc.).
Air Plants flower at maturity and will only bloom once in their lifetime (flowers can last several days to months). When nearing maturity the mother plant will start producing baby plants (pups). She will eventually die and the pups will repeat the life cycle.
Pups can be left to grow with the mother and over time will form a clump of plants. Alternatively, the pups can be removed when they are approximately one-third the size of mother and be grown individually.