The planted tank - How to grow plants underwater

Aquatic plants have adapted to live in a totally different environment to their land based relatives. Running through the plant, from the roots through to the tips of the leaves they have a system of air cavities, allowing them to maintain their buoyancy. Because they spend their lives in the supportive water they have far less supportive tissues when compared to the plants on land.

The one thing that all plants share and need, sunlight for photosynthesis. So this means that they need a green surface to absorb the right wave length of light.

In land plants they have a waxy cuticle to stop them drying out, in aquatic plants this layer is much thinner to help the plants absorb more of the gases.

The science bit

Okay so sorry here’s the science bit, I’ll keep it simple promise. Plants need a few things to survive

1. Photosynthesis; so plants take Carbon Dioxide and Water and give off Oxygen and store the rest as energy for life, growing, and in most of my tanks so something else can snack on them. This is driven by sunlight, or tank light, once the light goes out photosynthesis stops, and respiration (breathing) carries on, and contrary to what you heard, plants don’t breath carbon dioxide, they breathe oxygen and give off carbon dioxide like we do. It’s just that during the day photosynthesis takes in so much carbon dioxide and give out so much oxygen that the amount used for respiration is swamped. Hence why you must turn off any Carbon Dioxide source before the lights go out.
2. Nutrients; in the wild they get these from soil or from the water, and this can be a source of nutrients or you can provide these. The main ones are nitrogen, Phosphorus, Potassium and Sodium. Nitrogen for the most comes from your fish if you have them in the tank, as do a lot of the trace elements, you can fertilize your plants as well and I’ll cover that later.
3. Lighting as mentioned plants need the sun or at least tank lighting to photosynthesis, so to grow and survive they need lighting.
4. In general water can’t hold as much CO2 doesn’t let as much light through, and since most of us keep our tanks as clean as possible there may not be as many nutrients.

Not all plants that are sold in shops or online as being suitable for in the aquarium are in fact aquatic plants. Many grow along the margins of waterways in marshes and swamps, and may only be submerged for part of the year.

Submersed plants these plants grow completely underwater, and are adapted to live only underwater.

Emergent Plants these plants rise above the surface of the water and might only be submerged only seasonally.

Submersed plants will obviously do better if you plan to keep them under water, that’s not to say you can’t have the Emergent plants in tanks. If you have an open topped tank then they can keep their roots in the water, but let their leaves get out into the air. Just make sure nothing can jump or climb out.

CO2

So there are a number of ways of getting CO2 (Carbon Dioxide) to your plants.

• As a gas, this can be done with a DIY yeast based system, Aerosols which are good value, but don’t give you a lot of control, or the pressurised systems with valves and a number of methods for diffusion.
• Liquid CO2 Fertiliser. These aren’t as efficient as gas, but with the lower costs they may be suitable for smaller tanks.

Whichever system you use remember that CO2 can rapidly build up to toxic levels and kill the fish, especially if left on overnight or too much is put into the system, so it’s well worth putting your system on a timer to go off before your lights go out. To stay safe most people recommend keeping it under 30 ppm (parts per million).

Substrate

So substrate, the stuff on the bottom of the tank, serves two main purposes. First to anchor our plants to the bottom of the tank, and secondly and more importantly to provide nutrition. As mentioned you can provide everything they need via liquid fertilizers, but plants have millions of years of evolution saying they get everything through their roots, which need to be in some sort of soil.

Again there are a few options. You can buy complete substrates which are ready to go straight out of the bag, they provide everything your plants need and come in a variety of colours and grain sizes. You can go down the natural route and put in a base layer, often of specific brands of compost designed for land plants, or aquarium compost, then topped with a gravel or sand to stop it turning your water into a shade of mud. However it can be easy for the base substrate to work its way up and make a bit of a mess, and finally you can put in plain sand or gravel and put a capsual of fertilizer by the plants roots, but this can provide a very nutrient rich shot into plants roots. The tabs can be useful to revitalise an old substrate.

Some Plants to Consider

Nymphaea lotus; before the floating leaves form on the surface the leaves form underwater in attractive shades of green red and brown, with some purple spots. If you let them grow to the surface they can form beautiful fragrant flower.

Vallisneria Americana var. asiatica is a great plant for the background forming tall twisted green leaves. It is a vigorous plant that is easy to keep and propagates via runners

Hemianthus callitrichorides Cuba is one of the smallest aquarium plants in the world, with tiny millimetre sized leaves. It will spread rapidly and form a carpet across the foreground of your tank. A more challenging plant it can still look stunning.