Red plants add a vibrant touch of color to the planted tank. Generally, many red species require CO2, more specialized lighting and regular fertilization to truly show off their colors. However, there are a few species that will grow well even without CO2 injection. For a more in-depth guide for getting the best color out of red plants; read this link.
Alternanthera reineckii (these are grown in a CO2 injected tank). These will grow alright in a non-injected tank but the coloration will be more subdued. The undersides of the leaves will always be vibrant red while the topside takes stronger lighting and good growth conditions to show well. The best version to grow is Alternanthera reineckii 'mini' - it is smaller, more compact and redder than other variants. With stronger lighting and good conditions, it grows bushy and dense with no visible inter-nodes.
It is suitable for use in the mid ground of most tanks, or even foreground of larger tanks if pruned regularly.
They grow better in soil substrates. They are slow growers and vulnerable to algae on older leaves. They are a good test of overall tank stability and cleanliness. To read more in-depth on how to grow them well read this link.
In a non CO2 injected tank, Alternanthera reineckii 'mini' takes on more subdued tones.
Ludwigia sp. Red
Ludwigia sp. Red has become a popular plant in recent years. It is the reddest plant that can be grow well in non CO2 injected tanks and it one of the easiest red plants to grow well. Both the leafs and stems are completely red. It generally always has a strong, bold red coloration as long as it is healthy.
As a fast growing stem plant it is best used in the background - it is a relatively fast grower and require frequent trimming to be neat. Trimming off the tops and allowing the bottom portions to branch creates a denser, more attractive bush.
Nymphaea lotus 'zenkeri red'
The red tiger lotus has been in the hobby a long time. It grows quite large over time, with palm sized leaves (especially if given a deep, nutritious substrate in a CO2 injected tank). The leaves are a deeper shade of reds with dark spots. Nymphaea lotus 'zenkeri red' is a very hardy plant, and shows off good coloration even in non-CO2 injected tanks.
It is often used as a center piece plant due to its size, color and distinctive leaves. It can stunted through forced pruning to be smaller size to be used as a smaller mid ground plant.
It is an easy plant to grow that is very resilient.
Rotala rotundifolia 'red'
Rotala rotundifolia is a common stem plant that grows reddish under stronger light and low nitrate conditions. It is an easy plant to grow even without CO2 injection. In many tanks it looks more orange than deep red.
It is a stem plant that grows tall, however, it can be pruned to form nice dense background bushes.
To read more in-depth on how to grow them well read this link