Use water that has sat at room temperature for at least 24 hours so the chlorine has had time to evaporate. Chemicals in water right out of the tap can harm the flower.
Remove the peace lily gently from the soil of the pot with all its roots and carefully rinse off the roots.
Place the lily carefully in its new aquarium or other container of water. Some people like to grow lilies in an aquarium with beta fish.
Remove new flowering stems that have flowered and turn green, when you first get the plant. Future flowers will blossom better if the plant has been pruned. The roots of the peace lily will probably shorten as it adapts to the water conditions. Plants grown in water are usually top-heavy because they don't need the long heavy roots to find nutrients.
Repot or change the water if the tips of the plant start to turn brown, but be careful not to expose the roots to the air for long. Plants that have adapted to water conditions are sensitive to environmental changes. Let a new container of water sit out to let the chlorine evaporate before changing the plant water.
Move the peace lily to a larger pot if the plant stops flowering or appears cramped in its current container.