Eyeball & eye lense

I have created an eyeball and an eye lense at different layer. It is easier to select for texture and editing as the lense cover the whole eye ball. However right now I want to fix the eye to a fish. How can I make sure that the eyeball and eye lense move together to the eye hole location? and that they will not accidentally shift location (eg: eyeball coming out from eyelence?)

After I have created a fish, my intention is to do a simple animation. Do I need to move the eyeball and eye lense to the same layer as the fish for parent linking? Can anyone summarize the steps ? thanks

Ya, parenting is the way to go. So put it in the same layer. You can always use Outliner to select the lens you know. Selecting is not a problem. Just keep them organized in the Outliner by naming them properly.