3 Key pillars for aquatic plant growth

All aquarium plants need 3 things to grow. Aquarium plants that don't get enough of each will slowly deteriorate over time. The basic process that drives plant growth is photosynthesis, which in turn depends on 3 key factors.

LIGHT

Light is essential for photosynthesis, which drives all plant growth and metabolism. Photosynthesis only occurs during the light window and only works when sufficient nutrients and CO2 are available.

Matching the right amount of light to the plant's needs is intuitive on land, but often overlooked in freshwater aquariums. Are we planning to grow shade plants or open lake plants? The light requirements for the former are very different from the latter.

Most carpets need stronger light to grow well, just as the grass patch in a sunny outdoor lawn tends to grow better than the patch in heavy shade. Many red/colored plants also show more intense pigmentation under stronger lighting.

Many standard aquarium kits (tank + light + filter and/or other accessories) come with an aquarium light that doesn't produce enough light to grow more demanding aquarium plants.

This does not mean that expensive lighting is always good - there are many expensive lighting systems on the market with mediocre performance. On the budget end, choosing lighting can be tricky for beginners. LED fixtures are cheap and produce a lot of light for people who don't mind building their own fixtures, although they may not have the most aesthetically pleasing color rendition.

The light spectrum plays an important role in rendering colors well visually and has a significant impact on plant pigmentation for red plants.

Stronger lighting needs to be paired with the other two variables below, as the increased growth rates also increase the demand for nutrients and CO2.

There are a few important aspects to understanding lighting

What is the strength of a light?
What is the right spectrum?
Is T5 light or LED better?

Many picky species will require strong light to show good colors and form.

NUTRIENTS

All plants need a number of elements to grow well. Some of these nutrients may be readily available naturally in our tap water. Fish waste also provides some. However, many elements will still be missing if we don't add them. Basically, it is important to meet the nutrient needs of specific plants.

Optimal fertilization is one of the cornerstones of good plant health. We can achieve this through aquarium plant substrate (for rooted plants) and through water column dosing. Consistency is key.

Do I really need to add fertilizers?
How and what should I dose?
Is fish waste alone adequate?

Most colored plant species are quick let you know if your fertilization regime is not adequate - through pale leaves and thin stems. 

CO2

Carbon dioxide levels in natural lakes (especially in areas with abundant aquatic vegetation) are high (10-40+ppm) due to decomposition of organic material that forms CO2 gas, compared to the general equilibrium levels (2-3ppm) in a standard fish tank without CO2 injection.

Having optimal levels of CO2 in the tank has a major impact on plant health and growth quality, as ~40% of plant dry mass is carbon. This also directly affects the presence/absence of algae in a tank, as algae presence has a strong correlation to overall plant health.

Density, good plant form and color are all tied to having optimal CO2 levels.

By eliminating CO2 injection, we can still have beautifully planted tanks - if we pay special attention to selecting plants that grow well at lower equilibrium CO2 levels.

Which plants grow well without injected CO2?
How to set up a CO2 system for planted aquarium?