written by: Rob Charite
date: 19 June 2000

Light is the number one rule to secure a good plant growth.
They need light as an energy source for photosynthesis.
Most plants come from a tropical environment and there for don't do so well in natural day light. In the summer the light output is high and could be the cause of algae growth and temp fluctuations. In the winter light is poor and there for insufficient for proper plant growth. That is why we need artificial illumination. In the equatorial or subtropical area's where almost al our plants are originated from there is not much change in the length of day time. It lays some where in the 12 till 14 hours. This day /night ritme is needed for the plants in your tank as well that is why most people use timers . Plants have adapted to the light conditions for millions of years to their environment . A lot of tropical plants especially the red ones( which I like very much) need a very strong light source . Others like crypto's grow in the shade as well. So let's start with tl bulbs :_) How do we know how much light we need ? Well there is a simple rule of thump for that fi. With bulbs with a daylight spectrum output you'll need aprox 1 watt of lighting on 2 ltr's of water Energy saving bulbs do have I higher output spiral shaped bulbs provide more lighting then the conventional straight ones . These luminux bulbs have an extra light output from about 30 % there for you can use about 0.3 watts on 1 ltr water it is almost the same as hql bulbs only they don't have saving features :_)) 

Important note :
The light intensity decreases when it has to penetrate more deeper in your tank.
If we have a tank that is deeper then 40 cm the back plants mostly big ones or fast growers won't have any problems but the small plants in the front will decay due to an insufficient light source . if that is the case it is better to install a stronger light source or add an extra bulb.
This rules is also important when we filter over peat. The tannins released by peat do have the nasty habit of absorbing light and become no use to the plants.
Fish and plants will look natural if you use bulbs which imitate the daylight spectrum.
Photosynthesis occurs best in long waved red light or short waved blue light.
If you use the violet colored tl bulbs for a better plant growth f.i. the grow lux or fluora bulbs it is better to combine them with normal day light bulbs other wise your tank will look a bit unnatural.
In my humble opinion the strength of the bulbs will decrease when they get older after 6 month the output is half the strength of a new bulb so if you want a optimal plant growth it is better to change you bulbs every 6 month. especially the grow lux or fluora bulbs .
Note again : water with a high amount of nitrates needs more lighting.
In very clean water plants will grow under lesser light conditions ( so another reason to do the water changes !!!! )
In strongly polluted water the plants will stop to grow even with an optimum light source !!!
HQL bulbs can be used with tanks not deeper then 60 cm and especially for extra illumination (local) for demanding plants.
HQI bulbs are in my opinion not the best choice .
They do have I high output range but will let your tank look sterile
And last but not least :_
Please be careful with electricity in combination with water !!

Rob Charite & Walter Soestbergen

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