Rotala rotundifolia 'blood red SG variant' - this variant gets redder more easily than other Rotala rotundifolia variants, but is still not wide spread in trade (2021).
Rotala rotundifolia (variants include Rotala Colorata, H'ra, Ceylon, etc) is a fast growing stem plant that has been a staple of the planted aquarium world for a long time. It was first introduced under the erroneous name of Rotala indica around the year 1960, a confusion lasting to this date. The description of real Rotala indica can be found here - along with the explanation of how the names got mixed up.
It is tolerant of a wide range of water parameters and is an easy plant to grow; it does well even in aquariums without carbon dioxide injection after a period of adjustment. However, having sufficient light and CO2 allows it to grow with better density, and better coloration.
As stem plants they grow continually towards the light and will reach the top of even tall aquariums. If allowed to reach the water surface it will breach the surface eventually and grow emersed leaves and flowers. If stems are planted in a tank with high light and with a lot of space surrounding them, they will grow downwards and creep along the substrate. If planted in a more crowded area they will grow vertical more readily.
The plant branches more profusely when in fast growth mode, and grows more vertical with less branching if conditions are lean (either low CO2 or low nutrients).
Key success factors
Sufficient light (80umols of PAR+) to get good coloration
Avoid extreme water parameters (high alkalinity or uncycled tanks)
Higher nutrient levels coupled with CO2 encourages the plant to produce more side shoots to become bushy. In lean conditions an without CO2, stems are thinner and branch significantly less often. CO2 and adequate fertilization is required to grow the dense bushes you see here
How to get it redder
Stronger light (higher PAR values). While the plant can grow at lower light levels, having 80+ umols of PAR encourages better coloration. There is stronger coloration at 150-200 umols of PAR. This level can be easily hit at the higher areas of tank with good lighting systems.
Low nitrates (5ppm & below), while maintaining other parameters high
Rotala Rotundifolia is one of the plants that exhibit redder colors under nitrate limitation - meaning that the plant grows much redder if it is starved for nitrates. Over-do this and the plant will stunt. However, it is much more tolerant of low nitrogen levels than some other plants. When choosing to run low-nitrate levels in a tank, make sure you choose a selection of plants that adapt well to lean dosing conditions. You can read more on lean dosing on this page. It is more sensitive to water column NO3 levels than say ammonia in the substrate zone where coloration is concerned. The combination of having some ammonicial nitrogen in the rootzone while keeping the water column low in Nitrates (NO3) produces the best looking bushes overall. Being overly lean in Nitrogen in both substrate and water column can result in plants being thin, with much less branching, which makes it difficult to form dense bushes.
The Seachem nitrate test kit is one that has slightly higher accuracy readings for low NO3 levels for those keen on testing their water.
In the picture below, you see Rotala H'ra under Twinstar S series lighting. It takes strong nitrate limitation to induce such deep red colors - NO3 levels would virtually test 0 on test kits. Notice that it doesn't have that much branching, and overall bush density is average.
The same variant Rotala H'ra, when grown without nitrate limitation:
In this example, you can see clearly the exact point in the plant's growth stage where trimming was done together with nitrate limitation:
Another side by side comparison under same lighting:
There are many variants of Rotala rotundifolia; Rotala 'blood red SG version' seems to be a variant that does not require that much nitrate limitation to be very red - making it a better choices for most tanks. However, this variant does not seem to be commercially cultivated, and can be hard to find in most countries.
Rotala 'blood red SG' is used in the background in this tank. Top view is below
Combination of rich substrate, low Nitrates in the water column, and high lighting produces striking reds with good bush density.
Cut a couple of inches off the top of the plant and replant. New shoots will sprout from the inter-nodes on the stem left behind. If initial plant was healthy, you can cut off the majority of the plant, leaving stems a couple of inches in height and new shoots will still sprout from there.
Even if you want a dense bush, plant stems at least half an inch apart to leave room for side-shoots. There should be enough space in the bottom layers even if you want a dense canopy at the higher levels.
Rotala rotundifolia takes trimming well and will form dense bushes. It can be repeatedly trimmed for many cycles before requiring replanting of healthier/new tops. This makes it very useful for aquascaping where the ridge line needs to be kept at a certain height for aquascaping purposes.
Trimming should be done initially about 4 inches below the final height that you want your Rotala rotundifolia tops to be at. (unless you have planted your Rotala rotundifolia very sparsely to start with, then trimming further down allows more branching and density to build up) As the tops grow out, cut off the ones that grow faster than the rest - this allows the shoots below to branch and the canopy to gain density as it grows upwards.
After the canopy has been formed at the desired height, you should continually thin it by cutting off the tallest shoots and allowing new growth to take its place. This needs to be done very regularly (every 3 days or so). If the canopy rises as a whole after a long period of lack of trimming, then you have to do a reset as above; cutting the bush down much lower and growing out the tops again. As with carpets - if you want to maintain it long term, it requires very frequent trimming. Growing the tank under lean conditions also slows down plant growth as a whole and reduces pruning.
Over a long time (months), the older undergrowth will deteriorate to a point where it is unhealthy. You should then allow the top to grow out longer, then replant the bush as a whole.
Head here to find detailed steps on how to cycle a planted tank.