Knowing how much lighting your tank needs is one of those things without any definite answer. Most articles tend to flounder around for a bit and then give you some good (and practical) guidelines to go by, mainly because, for most plants it actually doesn't matter that much! But it is still good to know why, and that is what I will explain in this article.
For those who just want to get to the guidelines here they are:
Nutrients: One source of additional nutrients in your aquarium.
Low light: 10+ Lumen per Litre of water (0.7 Watt/L for Incandescent bulbs and 0.17 Watt/L for LEDs)
Medium light: 20+ Lumen per Litre (1.4 Watt/L for Incandescent bulbs and 0.34 Watt/L for LEDs)
High light: 40+ Lumen per Litre (3 Watt/L for Incandescent bulbs and 0.7 Watt/L for LEDs)
Lumens are a direct measurement for light intensity, and the best way to see how bright your light source is. Watts are how much energy the bulb uses, and varies on the efficiency of the bulb, for Incandescent bulbs you get about 1.5 Lumens per Watt and for LEDs (which are much more efficient but a little expensive) you get about 6 Lumens per Watt. Most lights should say on the sale page or on their box what intensity they provide.
Why doesn't it matter?
Most plants are great at adapting to their environments. Plants will grow differently depending on the lighting, nutrient and water conditions available to them. You will often see the same plants growing with longer stems, larger leafs or different colour hues even though it is the same plant, and that is because of the environment the plant is growing in. One great example is our Ludwigia Repens. Repens tends to grow larger greener leaves with red stems when under low light conditions, while growing thinner redder leaves under higher lighting, becoming more vibrant under stronger lights with the right spectrum and nutrients.
There are exceptions however, some plants are much more fragile, (Monte-Carlo for example needs extra CO2 to grow well) and may not be able to handle certain environments. It is best to check some info on each plant you get before you do but most medium to low light plants are usually fine with most lighting setups.
This doesn't mean you should have less lighting than the minimum low light requirement. Plants still need enough light at a strong enough intensity to be able to properly photosynthesize, and aiming around the 20 lumen per litre mark is my general recommended guideline for most aquatic setups.
All the plants we sell at Sydney Aquascapes are very easy to grow plants, and will thrive in most aquatic setups.
What does one source of nutrients mean?
Like most living things plants need a variety of nutrients to grow all the complex systems in their make up. As a general rule for most plants, and all those we sell, is as long as you have one source of additional nutrients you will not have a problem. A possible source could be; Fish (Their fesses and residue fish food), Aqua soil of any kind, root tabs or liquid fertiliser tend to be the 4 most common. As long as your aquarium has one of these your plants will be fine in most circumstances. CO2 does not provide any nutrients for the plant, it only increases growing speed, and only alongside good lighting. You can imagine adding CO2 being similar to making it easier for us to breathe, it won't make us more healthy directly and we still need to eat, but it will make growing and doing what humans do much easier.
How do I know when I am missing something?
If your plants are not growing, wilting and loosing their leaves the most probable cause is not enough lighting. If that isn't the case it could be many reasons from water condition to infections to your fish eating them. You would need to research further. If your plants are growing but something seems wrong, such as them being off-colour or not fully developing, this usually means you are missing a critical nutrient in the aquarium for them. Do a little research on the symptoms of certain nutrient deficiencies in the problem plant and see if you can get a source of it in your setup.
I hope that covers all the critical info you need about lighting! Feel free to ask us any questions you have.