Plants make food to feed themselves through a process known as photosynthesis, which occurs when green plants use sunlight to synthesize nutrients from carbon dioxide and water.
For plants to be able to feed themselves, they need to get enough sunlight. There is no care that can make up for a plant getting too little light, and plants growing in aquariums are no different.
How many hours of light do aquarium plants need?
Low-light aquarium plants will usually require between eight and ten hours of light per day, while medium-light aquarium plants generally need around ten hours. High-light plants require between ten and twelve hours of light per day.
Unlike plants growing in the ground or in a pot, aquatic plants growing in glass tanks will receive sunlight that has been magnified through the walls of their aquariums.
The glass creates a greenhouse effect, trapping in heat, warming up the tank’s water and magnifying the intensity of the sun on plant leaves.
For this reason, most aquarium owners prefer to place their aquariums in indirect light and to use fluorescent, incandescent, metal halides, or LED grow lights to stimulate plant growth.
When it comes to choosing an artificial grow light, it is wise to consider how much heat these will produce. Incandescent, fluorescent and metal halide light bulbs all produce a significant amount of heat, for which reason many aquarium owners prefer to use LED lights.
Just as with plants growing in any other environment, no two plant varieties have the same light requirements.
Just as there are high-light, medium-light, and low-light houseplants, there are low-light, medium-light, and high-light aquatic plants, and they need to be catered to according to their specific needs.
For this reason, it is also important to pay attention to plants’ light needs when choosing which plants to grow in an aquarium together. It is difficult to grow plants with vastly incompatible light needs in the same space.
While it can be done by creating a clever lighting system using timers and multiple LED light bulbs, you have to be ready to invest the time and money into doing so if you want any of your plants to be able to photosynthesize optimally.
Things to consider when determining how much light plants growing in aquariums need
How you set up your plants’ artificial lighting will be further impacted by several other factors, including how bright the room is that your tank is in, what species of fish live in your aquarium, how many plants are in your aquarium, how much algae is in your aquarium and how large your aquarium is.
The answers to all of these questions will impact how you set up your plants’ lighting. Here are a few general things to keep in mind when assessing the light requirements for your planted tank.
If you are keeping your aquarium in a brightly lit room, you will not need to give it quite as many hours of artificial light as you will if you are keeping it in a dark room.
If your tank is relatively close to a window and is regularly receiving bright, indirect sunlight, or even a few hours of direct sunlight, you may be able to knock an hour or two off the recommended hours of light for a tank owner. For clarification about what these light descriptions mean, have a look at this essential guide to houseplant light levels.
A good general rule of thumb when it comes to achieving compatibility between the light and heat requirements of your fish and your plants, is to match plants from one climate with fish from the same one.
The more plants are in your aquarium, the more carefully you will have to assess whether all of them are in fact getting the sunlight they need.
It is possible that they are either having their light blocked by other plants or are simply too far away from your grow light’s spread to get enough light.
If this is the case, you may need to purchase another grow light to set up in the tank to ensure equal light exposure amongst all your plants.
Alternatively, you may need to arrange your tank so that those plants with higher light requirements are closer to the grow light(s) and those with lower light requirements are still receiving enough light, despite being further away from the light source(s).
Take care that floating aquarium plants are not blocking other plants from getting light.
A further thing to consider when setting up your aquarium, is that you should not plant your aquatic plants too close together, as this will inhibit the spread of light and prevent all your plants from accessing light.
Your fish should be able to comfortably swim through and amongst your plants, as this means there is enough space between them for the light to reach all parts of your plants.
The more algae-prone your plants are, the less light you should try to expose the water in your aquarium to.
While you should not compromise your plants by not giving them enough light, you will need to strike a fine balance between providing your plants enough light for them to healthily photosynthesize and avoiding giving your tank so much light that it becomes a cesspool for algae.
If algae is allowed to grow unchecked, it can seriously compromise the health of your plants.
If you notice that algae is building up, try reducing your light levels and see if you notice a difference. Once the algae growth subsides, you can try slowing increasing the hours of light exposure you give your plants again.
It is easier to minimize the growth of green algae in tanks with low-light houseplants such as marimo moss balls and java moss. It can be tricky to do so in tanks with high-light plants, where the light levels cannot be reduced too much.
If you are having algae problems in a tank with high-light plants, try keeping the grow lights closer to your individual plants and reducing their brightness slightly.
The larger your aquarium is, the more light you will need to use and the more lumens (a measure of the strength of a light) the lights you use will need to have.
As a general rule, you need grow lights with 10–20 lumens per liter for low-light plants, 20–40 lumens per liter for medium-light plants, and over 40 lumens for high-light plants.
Keep in mind that tropical plants will require consistent light levels throughout the year, while cold water plants such as Anubias will require less light in the winter than they do during the summer season.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.