'Red plant' is a generic term I use for non-green plants in this case. If you look carefully at the picture above, each species has different color tones. Too many hobbyists make the mistake of thinking that more fertilizer is better. Not necessarily so. I dose less than 0.1ppm of Iron a day in my tanks for example.
This is probably one of the most common questioned asked by aquarists. Primarily, it is a difficult question to answer because different red plants respond to slightly different variables, so one size fits all answers seldom work out.
Plants require an entire set of nutrients to be healthy and look their best; many folks buy into the myth that dosing more iron automatically makes red plants more red, but this is not so. While iron is necessary for the general health of plants and healthy plants develop pigmentation better; dosing iron in higher amounts than what is required does not result in stronger pigmentation. On the other hand, other elements, including phosphates & magnesium, are often under dosed by aquarists because many commercial fertilizers are short of these components.
The baseline for growing red plants well is that they must be healthy. Many aquarists overlook the basics - unhealthy plants rarely display nice aesthetics. This means understanding the requirements of a regular fertilization regime, good CO2 control, flow, substrate management, plant husbandry. Folks tend to gloss over this part, and assume their tanks are "doing well". However, I find that it in this area most planted tank aquarists have a lot of room to improve in.
Aquarists tend to be weak in their plant husbandry & maintenance regimes, while being overly focused on nutrient dosing or PAR values. The former skill set has as much impact on plant growth outcomes as the later. Husbandry in this case refers to trimming, replanting techniques, gauging when over-crowding occurs or when a plant prefers more or less flow, gauging whether substrate is becoming compacted or depleted in nutrients and changing fertilization tactics as necessary.
After plant health is taken cared of, we can examine the few specific impact factors that affect red plant pigmentation specifically. Different plants respond slightly differently to different impact factors.
Rotala macrandra mini Type 4
This works for all plants, the more light you have over the tank, the stronger pigmentation you will get. The downside of pushing light values high is greater chances of algae in the tank. Having healthy plants and a clean tank allows one to use higher lighting without facing algae issues; this is why getting maintenance and plant growing basics right is important. Around 100umols of PAR on the substrate is firmly in the high light category where one would expect very good density and coloration for most colored plants. On average, most of such plants can be grown in tanks with much lower PAR (50umols +), but coloration and density will only get more impressive towards the top of the tank where light is stronger.
AR mini (left) growing in a tank where substrate PAR is around 100+. However, if you want Proserpinaca palustris (right) to color up well, you will need much more light than that - even in high light tanks, it usually only gets much redder closer to the top of the tank/lights where PAR values are 300++.
Many crypts such as the Cryptocoryne 'flamingo' (left) from my tank have good coloration and are easy to grow. Echinodorus species such as E. Rubin also easily show good colors as long as they are healthy and under strong light.
Having strong red/blue spectrum in the lighting system has a dual effect. The first effect is a visual one; if you have stronger red lighting, it lights up red plants better. Many commercial LED diodes lack red spectrum, which tends to wash out red tones. The second effect is that stronger red/blue lighting stimulates pigmentation in many plants, this means that many red plants will both grow redder and look redder under strong red spectrum. This is why many T5 sets with specialized pink grow bulbs grow red plants better than white commercial LED lights which tend to lack red in their spectrum. RGB LEDs have much better spectrum coverage and color rendition compared to white-only LED units. This is why Kessils and many other commercial LEDs grow red plants poorly.
If you use too much red lighting and cast a monotone redness over all plants the tank will artificial. I prefer a more rounded spectrum - in my T5 array I use red tubes with a mix of orange and blue tubes. Good color rendition in a tank is when you can see colored plants in the tank in all their different shades of color and not just a uniform red. This is also how you differentiate between tanks that just have red saturated lighting and tanks where individual species are grown well enough to show off their individual color tones.
My Bucephalandra brownie ghost in a tank with more blue heavy lighting (left) vs more neutral tone lighting (right). Iridescent plants are especially affected by the color rendition of the lighting system. Make no mistake; red plants definitely look redder if there is plenty of red light to reflect - but it is a balance to get an outcome that looks striking rather than fake.
Nitrate limitation involves dosing less nitrogen into the tank compared to other nutrients. This lack of nitrogen delays chlorophyll development in certain plants and allows them to grow redder. This works only for some specific species (Rotala rotundifolia, ludwigia arcuata etc) and does not have a significant impact on others (Rotala macrandra & variants, Ludwigia pantanal, Althernanthera reineckii). Limiting nitrogen should be done carefully as plants that favor richer nutrient levels may stunt if levels dip too low. (for example, Ludwigia pantanal).
To do this effectively in a tank, planning should be done beforehand. Whenever one is going lean in nitrates in the water column, it makes sense to use a rich substrate to compensate. Nitrate limitation seems to be impacted by water column dosing of NO3 rather than substrate. For species that require low nitrogen to be red, typically you would aim for 5ppm or less residual NO3 levels in the tank's water column. Ammonia rich aquasoils are perfect counterparts to this strategy as they provide a store of nutrients that plants can draw on, albeit in a slower mechanism than water column fertilizers. Feeding N through the root system seems to allow such plants to have the effects of N limited increased coloration without stunting. Most plants actually adapt well to lean water column dosing as long as there is rich substrate. Every single aquatic species I've seen can be grown using rich substrate + lean water column dosing. However, richer water column dosing produces more robust forms in many species.
Plant selection/combination also plays a part. If you plan to grow very red Rotala H'ra (or other variants of Rotala rotundifolia), which requires very steep nitrogen limitation - with water column NO3 levels measuring near 0 for extended periods (weeks), then you should not attempt to grow plants that grow optimally in richer nutrient levels in the same tank.
Most of the plants that respond well to nitrate limitation (examples are listed below), also respond well to stronger red/blue light spectrum and stronger light. This means that you can choose a middle ground; where nitrates are low but not allowed to bottom out (say 5ppm), paired with stronger, red/blue shifted lighting, to get reasonably good color out of plants that require nitrate limitation to get red.
Rotala H'ra under slight nitrate limitation on the left (5ppm NO3 in water column, but under stronger lighting) vs H'ra under strong nitrate limitation (0ppm NO3 in water column - plants are mainly root fed through aquasoil). There is no way to get Rotala rotundifolia variants this uniformly red without letting NO3 bottom out for an extended period.
A side by side comparison picture of Rotala H'ra grown under two different nutrient regimes, under similar PAR light values:
There is some rumor (as usual) in hobbyist circles that Tom barr refutes the effects of nitrate limitation on plant pigmentation. The opposite is actually true - he published examples in early years that demonstrated that nitrate limtation works for certain species. However, he generally does not recommend it as an approach due to the risks of growth bottoming out due to going overly lean in nitrogen.
In his own words:
Another reminder of the basic point that all plants need to be healthy before they can display strong pigmentation. This means good control of CO2 and having a well-rounded fertilization approach.
There are a few easy red plants that turn easily red as long as their fertilization needs are fulfilled and PAR values are not too low (50umols +); this includes the Nymphaea zenkeri 'red' (below, left), Ludwigia sp.red (below, right), Echinodorus 'rubin'&variants. The various colored Cryptocorynes are also easy plants to grow in general: C. Undulata red (reddish/brown), C. flamingo (pink/only TC is difficult), C. 'Hobbit' (Brown), + others. These are the easiest red plants to grow for beginners that are just starting out. Many other plants also have guaranteed red undersides of leaves (even if their tops are greenish); Althernanthera reineckii species, Ludwigia repens/palustris.
There are a couple of plants that are more difficult in terms of requirements, but always display good coloration if you can grow them. In sequence below: Ludwigia glandulosa (purple, not too hard), Rotala ramosior 'Florida' (purple, a bit harder) and Rotala mini-butterfly series (more sensitive).
Most other red plants require at least high light levels and a decent spectrum (more red/blue) for stronger coloration. These are plants that are generally reddish to start with, but grow much redder depending on how heavily red/blue shifted the light spectrum is and often require quite a bit more light than the average tank to color up optimally. Examples: Rotala macrandra & its many variants, Ludwigia senegalensis, Althernanthera reineckii, Didiplis diandra, Persicalaria 'Sao paolo' (purplish)
Then there is the group of plants that show redder pigmentation due to Nitrogen limitation/low nitrate growth conditions (which delays the development of chlorophyll). This category of plants grows significantly redder when deprived of nitrogen Examples: Rotala rotundifolia red and its many variants (Rotala colorata, H'ra, Yao yai, etc), Ludwigia arcuata, Ludwigia brevipes, Hygrophila araguaia, Hygrophila pinnatifida, Limnophila aromatica/hippuroides (topside). Many other plants that are partially red will have larger red portions under limited NO3 as well; Eriocaulon quinquangulare, Trithuria lanterna. However, most of these plants also color up better when exposed to high PAR and stronger red/blue spectrum. So having not too lean water NO3 levels in the water column, coupled with good lighting is somewhat of a middle ground that is easy to hit for many tanks.
Certain species such as rotala rotundifolia variants and Ludwigia arcuata only get very red under nitrate limitation and they will not achieve high degree of redness no matter how much light you throw at it without NO3 limitation. If you plan to grow these species to very high degree of redness, planning a tank where the dosing approach is lean as whole works better. This also means avoiding using hungry species that favor higher nutrient levels in the water column in the same tank (they may not grow to be in best form).
Ludwigia arcuata and ludwigia pantanal (left); not the ideal tank partners. Ludwigia arcuata gets much redder with low NO3 levels; whereas Ludwigia pantanal stunts easily if conditions are overly lean. Hygrophila pinnatifida (right) gets significantly redder with Nitrate limitation - otherwise, its a green plant even under higher lighting.
THE LEAN DOSING TANK
In these tank systems, plants are mainly root-fed - this is typical of ADA style systems where the water is very lean in nutrients (especially nitrogen) but the soil is rich. Nitrate limitation occurs naturally over time if one follows typical ADA dosing approach, which is very lean in water column NO3.
This approach is best for hardscape heavy tanks with sparse planting and large unplanted spaces or scapes where there is generally low nutrient draw (where majority of plants are slow growers such as Java fern, anubias or Bucephalandra). This approach keeps the water column lean which means less incidence of dust algae on hardscape/glass, reduces dosing requirements (save cost + time), and is suited for plant combinations that get good color under lean conditions. Rotala rotundifolia (and its many variants) and ludwigia arcuata (below) are a staple in many ADA style layouts and they grow to good form with good color with this approach. The same plants become super weedy in EI style rich dosing tanks, while being more green/orange rather than red. (and often leaving hobbyists puzzled: "I'm dosing so much, why aren't my plants red?!") Iwagumis also do well under this approach. Many plants also grow in smaller growth forms under lean dosing; myriophyllums that grow huge under EI stay more reasonably sized in lean dosing tanks. This is useful for keeping plants under control and adhering to contour designs for aquascaping.
The main disadvantage of running such a system is that one is dependent on the rich soil to grow hungry plants. Even carpets do not grow well without a soil substrate in this scenario. If one is dependent on substrate richness for nutrient delivery, then addition attention must be taken to monitor substrate quality. (and less hobbyists have experience in this angle as it is a topic that is rarely discussed). What happens is that these ADA style tanks with aquasoil often start out with strong growth, but peter out over a year. By that time, many aquascapers just rescape with the addition of new soil. This does not mean that such an approach has no long term sustainability - substrates need to be enriched over time, compacted layers loosened and excess organics siphoned away. Many aquascapers fail at managing substrate health, thinking that they can just leave it untouched forever, as this topic is almost never discussed (unlike water column dosing which is a popular topic).
With a very lean water column, hungry plants may temporarily stunt during replanting cycles as their root zone takes time to develop. This means that the aquarist should prune tops off whenever possible, and allow bottom portions of stems to regrow as replanting tops requires time for the plant to re-establish the root system. Aquascaping styles that are partial to frequent replanting of tops may not fare as well with this approach.
THE MIDDLE GROUND (slight NO3 limitation)
One does not always have to choose an extreme system. I like my tanks to be NO3 limited, but not to an extreme extent. I maintain low NO3 levels - enough to get good color (but not super redness) out of plants that require low NO3 to be red, however, this middle ground also allows me to grow any species of plant on the market to good form. I find that most colored stem plants work well under this system - their growth speed is more controlled which allows me to maintain the look of a tank with less frequent pruning work. You can read more on the exact dosing levels I use on the page on dosing systems. While I maintain low NO3 levels in the water column, I counter balance it by using rich substrate with frequent additions of root tabs. Growth of plants is more controlled, which reduces pruning work as a whole.
The main difficulty with this approach is similar to the lean dosing approach above; it requires good soil substrate management. Water column-wise, what is lean for one tank might also be too lean for another - this requires the aquarist to have some experience in interpreting plant growth forms; this can be easily learned over time by the observant, however, is probably difficult for beginners. Maintaining a low but consistent macro level also requires strict discipline in dosing and monitoring of plant health.
RICH WATER COLUMN DOSING APPROACH (EI style tanks)
This is style is suitable for tanks with heavy plant load that can make use of the rich water column levels. If you have a plant dominant layout, water column dosing reaches all plants without prejudice as to whether they found a good spot on the substrate. Subsequently, this approach does not require one to pay as much attention to substrate richness and allows one to manage a planted tank without fussing with soil. Many plants grow more robust, larger, and more quickly with rich dosing. If you enjoying trimming plants to sell this is an advantage. This approach gives the fastest growth and allow faster completion of aquascapes if there is time pressure to complete a project.
This approach is ideal for propagating plants at fast speed and aggressive growers do better than in lean tanks. Delicate samples (due to transport or poor stock or attempts at propagating difficult species) also have better survival chances as they can attain nutrients more easily.
This approach is also particularly suitable for aquascaping layouts where topping and replanting is done often; plant can take in nutrients through the water column before needing to fully establish new root zone. Joe Harvey's (burr740) tank below demonstrates good water column dosing technique with an inert substrate.
The main downside of this approach is the need to manage water parameters well (more complex than what it looks on paper), and to dose regularly. I find that for sparsely planted layouts with low nutrient draw, having an extremely rich water column introduces instability - while nutrient themselves are rarely the trigger for algae blooms; if you do trigger algae blooms (and most hobbyists will at some point in time), they are more severe in tanks where water column dosing is done heavily compared to tanks with a leaner water column. If you are doing a simple Iwagumi or hardscape heavy scape with sparse planting, I find the instability of having heavy water column fertilization as a significant drawback.
Tom Barr's 120-Gal tank above.
Choose plants and a dosing methodology based on your aquascaping goals. If you think about it, in the fish world folks accept that hard water Cichlids should never be kept with soft water Kilifish. Is it that hard to accept that plants do well under different conditions? Though the rules are not as strict in the planted world (many tanks can grow most species) - our plants come from various different regions and habitats in the world. Planning tanks where species that do well together are grown under similar conditions certainly goes far in impacting outcomes.
Head here to find out more about optimising CO2 and flow.
Head here to find out how to read PAR values.
Head here to section on detailed care guides to aquarium plants.