Your fish need light! They're used to being in the wild, where it's daylight for like 12 hours. It'll stress them out if they only have 2.5 hours of light per day.
It's not expensive at all. I didn't even notice a change in my electric bill after i set up my tank! Do you have a job? Offer to mow the lawn or do some house chores to make up if there's a difference?
and with florescent lighting, it doesn't use much electricity at all.
It's not just about them seeing. It has health aspects too. If you want corals, you'll HAVE to leave them on all day long.
That's an expense of having a fish tank. I've already prepared for that, just like i have to buy food, and salt, and testing supplies, and all of the other not-so-fun stuff that comes with the responsibility of caring for my tank.