Under the HOBBYbrand, we market T8 fluorescent tubes and high-pressure mercury vapour lamps for lighting aquaria. Light is especially important for plants in freshwater aquaria. This is because they not only have a decorative function; rather, they also fulfil an essential need for fish. Under exposure to light, plants use photosynthesis to turn the elements carbon dioxide and water into carbohydrates and sugar - the plants' principal components. A by-product of this is oxygen, which is essential for fish and other animal organisms. Algae are in constant competition with plants for nutrients. In a healthy aquarium with low nitrate and phosphate values, sufficient plant stock and the right fertilisation and lighting, plant growth holds the spread of algae in check. But if the lighting changes - even when the provision of nutrients remains the same - the algae can quickly gain the upper hand because they can adapt to the light ratios better.