There are some clear benefits to timing your fish's lights. For one thing, every animal needs a predictable schedule of night and day in order to enjoy the best health, same as us.
I have used this particular timer for about 8 years to control the lights on several things, including my fish tanks. If you thought you couldn't afford a timer, think again:
Here is a digital one for not much more money, though the analog one actually got slightly better reviews:
These aren't fancy, but they are hard to beat for the price. You will need to re-adjust for power failures occasionally (which takes me about 15 seconds to do), but they sure beat having nothing at all if your schedule makes it hard to provide your pets with regular hours.