Xenia will thrive in just about any condition. However, I would put them towards the top of your tank. They tend to move towards the light on their own and will spread everywhere along the way. Also, try to keep them isolated, othwise they will spread everywhere. If you bought a pulsing Xenia, make sure to keep your idodine levels up. There is no real proof, but people (me included) seem to think there is a connection between the pulsing and the idodine. Hope this helps.