Light plays a very important part in the aquarium , especially in freshwater tanks where there is living plant growth. By sustaining plant growth, light also plays an indirect part in keeping water conditions at their best. As plants photosynthesize under the influence of light, they remove carbon dioxide from the water.

Although you can use ordinary light bulbs in the hood (some aquarists still believe these grow the best plants) they are inefficient, produce a lot of unwanted heat and generally have a shorter life than fluorescent tubes.
With the basic lighting fittings supplied with the normal reflector/hood, there will be adequate light for you to see what's occurring in the tank, and the fish will have enough light to go about their business.
However, to ensure good plant growth (for the beneficial reasons stated above) it may be necessary to double the amount of light. This can be done quite easily by adding another fitting and starter gear (if fluorescent tubes are used) or you could try adding a reflector to the tube you've already got and watch for better results.

Excessive light will only lead to rampant algae growth so make sure you have enough plants in the tank to take advantage of all the extra light 'energy'; strong-growing plants will out-compete the algae for the light.

Fitting a cover-glass will cut down evaporation losses and stop dust getting into the tank (and fishes from getting out) but keep them spotlessly clean so that all of the light generated gets to where it's needed most - in the tank. To maintain good plant growth, it is also necessary to change the fluorescent tubes regularly - probably once a year - to keep the light levels at their optimum.