how many do you have in the tank? they may be super nervous like star rider was saying, and as opposed to a cover question perhaps it is a stocking question.
I have 7 lemons in a tank with little cover, and instead of "trying not to be seen" they are very active and constantly schooling. a smaller number of tetras will always feel nervous, and the larger the group the more comfy they will feel (in fact I've been thinking about adding a few more to my school).