Why nutrient level stability matters for planted aquariums

Plant responses to the changing environment

While plants appear static to human eyes, plants are constantly sensing and making adjustments to environmental conditions. Aquarium plants constantly make changes to match available resources in the environment. Obvious plant responses that can be observed in aquarium plants include stem elongation when the plant is deprived of light and growing submerged formed leaves that are more efficient in gaseous exchange when the plant is grown submerged. Plants make such adaptation changes to optimize usage of available environmental resources such as light, carbon dioxide and nutrients.

Many adaptation changes occur internally in the plant's biochemistry.  Plants tune up their growth hormones when nitrogen is plentiful and produce larger leaves along and grow faster. When nitrogen availability in low, many plants take on smaller forms and may also delay chlorophyll production, which leads to redder leaves on some species.

Gradual/small adaptation changes in the aquarium happen invisibly most of the time and hobbyists pay little attention. Many easier plant species are extremely adaptable and can weather significant changes without symptoms.

However, when the adaptation change is big enough to illicit a stress response in plants - that is when hobbyists take notice. Some species such as Rotala macrandra produce stunted tips when undergoing adaptation stress. The apical bud tip dies off, however, the plant may produce new side shoots that survive. In other species, such as Cryptocoryne species or Bucephalandra, it can manifest as melting of leaves.

Older leaves have limited capacity to adapt to new environmental conditions. Older leaves are often abandoned as plants adapt to new conditions by producing new leaves with internal machinery that is optimized for the new environment. As plants let go of defending their older leaves and channel energy to new growth, algae attacks older growth. This is most obvious when transitioning emersed grown plants to submerged growth - the emersed grown leaves are eventually all discarded as the plant produces submerged grown leaves. 

After plants are added to a new aquarium, older leaves that are not programmed for the current environment are abandoned. The plant will produce new adapted leaves that remain algae free. This results in a mix of algae-free new leaves along side older leaves that attract algae. The correct solution in this scenario is to prune off the older leaves and leave the plants to settle in while keeping the environment stable. The wrong approach is to tweak parameters values further - inducing even more adaptation changes in the plants and prolonging the problem. This is why many hobbyists find that their planted aquariums settle in smoothly only AFTER they give up and stop making changes to the aquarium. Many tanks suffer from iatrogenesis.

iatrogenesis- a term for when the action that is meant to help, harms the patient instead.

Experienced hobbyists have a tremendous edge in managing new tanks because they know a certain set of parameters will work and can leave the plants to stabilize with time rather keep tweaking parameters. Newer aquarists are often left guessing whether could they have done something different to improve plant growth and often end up destabilizing the system by making too many changes.

Differentiating "nutrient deficiencies" from unstable nutrient levels

Plants suffer from nutrient deficiencies when they do not receive enough nutrients for growth. Plants suffer from re-programming stress when they receive enough nutrients for growth, but nutrient levels are unstable.

Nutrient deficiencies

• Overall growth is weak and noticeably slow
• Characterized by smaller sized new growth, poor formation of new leaves
• Plant size tends to shrink over time
• Tends to take some time to show up, as plants can store nutrients for future use
• Change is gradual, not sudden

Unstable nutrient levels

• Growth continues to be strong
• New growth is normal sized, well colored
  
• Algae attacks plant despite positive plant growth
• Plant growth or algae issues show up suddenly rather than gradually
  

Rotala florida shows good form and color. A 10ppm nitrate level spike in this tank caused enough adaptation stress that algae attacks the plant. Folks think that throwing more nutrients always make plants healthier - and this only works up to a point. Nutrient level stability is also important to prevent plant reprogramming. 

Algae on older growth on Micranthemum 'Monte carlo'. Is the plant deficient? Or is just older growth being abandoned while new growth continues to be algae free? Plants can go through adaptation stress due to a large variety of reasons - CO2 fluctuations, biological instabilities etc. Not all forms of adaptation stress needs to be solved by increasing nutrients. More often than not, if new growth is well formed - just pruning/uprooting old growth and replanting new growth is sufficient to solve the issue.

Stable nutrient levels are the key to preserving older leaves and maximizing algae free old growth. Many plants also only color up fully in stable growth conditions.

Does one nutrient matter more than another?

Not every factor that influences growth causes adaptation stress where aquarium plants are concerned. Light levels for example, have a large influence in growth but most plants seem to be able to navigate relatively large changes in light levels smoothly. Similarly, upward changes in CO2 levels seem to have no negative impact on plants whereas downward changes in CO2 levels do induce significant plant stress. 

Not all nutrients have the same influence on growth. Besides carbon availability, nitrogen availability is what stimulates plant growth the most. Perhaps this is because plants use more nitrogen for growth than any other nutrient besides carbon. In systems where nitrogen is readily available, plants tune up their growth rates. When nitrogen access is lean, plants grow significantly slower and often with smaller growth forms as well - this happens even if other nutrients are presented in excess. 

While all nutrients are important in some way, to prevent this article from sprawling to infinite length, we will be discussing mainly nutrient levels stability using nitrogen levels as an example. Nitrogen is available to plants through ammoniacal nitrogen in aquasoils and through nitrates/ammonium in the water column. Nitrogen spikes through the substrate route causing issues is rare and usually limited to DIY osmocote users who use terrestrial osmocote that release their contents too quickly. Water column nitrate levels is a factor with much more impact.

A well planted, CO2 injected aquarium can uptake anywhere from 1 to 8ppm of nitrates over a 24 hour window. For a moderately stocked aquarium, about 1-4ppm of nitrate is produced from fish waste daily. Lightly stocked aquariums with light feeding can produce just 0.5ppm of nitrate per day. Additionally, most aquarists add nitrates through fertilizing the water column.

Let us example the way nitrate (NO3) can build-up with some simplified examples. (ppm) denotes Parts Per Million. In most tanks, a swing of 10-15ppm NO3 is enough to cause plant reprogramming and a spike in algae.

Nutrient dosing and nutrient build-up in aquariums

Tank Example 1. 

Tank starts week at 0ppm NO3. Daily dose of 2ppm NO3. Plant uptake rate of tank is 2ppm NO3. Tank measures consistently 0ppm NO3 residual levels in the water column. This is an example of lean dosing, leading to consistent residual levels of nutrients (zero or near zero residual water column NO3). This is an example of zero bound stability as the amount of nitrogen available to plants daily is consistent.

Tank Example 2.

Tank starts week at 0ppm NO3. Daily dose of 4ppm NO3 a day. Plant uptake rate of tank is 1ppm of NO3 a day. This causes a net daily accumulation of 3ppm NO3 a day. Tank starts week at 0ppm of residual NO3 a day, rising to 21ppm of residual NO3 after 7 days. This is an example of heavy dosing, but inconsistent residual nutrient levels. The plant that started out adapted to growing at 0ppm residual Nitrate levels now will start adapting to high nitrogen availability at the end of the week due to spike in NO3 levels. This will induce an steep adaptation response in some plants and cause algae.

Tank Example 3.

Tank starts week at 15ppm of NO3 in the water column. Daily dose of 3ppm of NO3. Plant uptake rate of tank is 3ppm of NO3 a day. Tank measures consistently 15ppm of NO3 in the water column. Plants adapt to having 15ppm NO3 in the water column. This is an example where residual water column nutrients are high, but consistent. This is a stable growth environment.

Zero bound stability/nitrate limiting tank (Tank example 1)

This is one approach that is easy to execute to maintaining consistent water column nitrogen levels: using a rich substrate while dosing more on lean side. This has given rise to the observation that some plant species seem to be easier to grow in rich aquasoil substrates. However, the mechanism on which this works is that it is easier to have low stable water column nutrient levels when the plants can feed from a rich substrate.

We use an example where plant uptake rates match or exceed fertilizer dosing rates (using nitrate as a proxy) on a daily basis. Such a system will have no net accumulation of nitrates in the water column. This means that any nitrate dosing is fully taken up by plants. Nutrient build up = Fertilizer dosed - Plant uptake. The build up rate for Nitrates in this tank is 0ppm daily, so the nutrient buildup bars below are at the zero bound.

The downside of running such a system is that less competitive species could be starved of nutrients if the dosage falls far below what is required. Aquarists have to observe their plants closely to detect whether they are under-dosing. The way to get around this is to use a rich substrate where plants can get additional N through the substrate if they require.

ADA famously runs mostly zero-bound systems coupled with their rich aquasoil. Plants in such setups moderate their own growth rates naturally as the tank gets more grown in as more plants compete for available nutrients. This gives longevity for many layouts compared to aquariums where all plants are consistent well-fed and outgrow their surroundings quickly. Plants in lean systems grow more slowly and also result in less maintenance in trimming and replanting. Zero-bound systems are very stable and easy to manage in the short run, especially when paired with easy to grow plants. This is why commercial systems such as the ADA system have gained widespread popularity. 

The main downside in such aquariums is that pickier or more high demand species will be out-competed by species that can scavenge nutrients more aggressively. One observation is that while many ADA layouts start out with many interesting species, the longer the tanks are run, the fewer species remain as the weaker species get out-competed. Without careful monitoring, many plants get weakened over time as the layout matures and the competition for scarce nutrients increases. This can lead to a tipping point where the tank crashes easily due to all plants being in a chronically fragile state. However, due to the long time horizon such crashes take to play out, many fans of such an approach just make use of the opportunity to refresh the layout.

These downsides can be countered by aquarists being more active in monitoring plant growth of individual species, and compensate by dosing more or actively enriching patches of aquasoil where necessary. Many of the 2hr Aquarist aquariums are run on the zero-bound approach as well, but due to more active management we can grow a much wider range of demanding species well compared to what ADA gallery can manage.

In reality, up take rates for plants can mismatch against fertilizer dosing rates. Many folks may also want to dose at least slightly in excess to cover any fear of under dosing. By dosing slightly more than the average uptake rate of our tanks, we can easily manage nutrient levels by doing a large water change reset at the end of the week. A more realistic nutrient accumulation chart would look like the one below where plant uptake fluctuates within a zone even when nutrient dosing is kept consistent. A large water change (80%) is done each week to reset levels. If you are always over-dosing slightly in excess, doing a large water change each week (much more than 50%) is highly recommended to reset levels. Nutrient build up = Fertilizer dosed - Plant uptake (cumulatively). The build up rate for Nitrates in this example varies from 0-0.8ppm per day.

In this above example, accumulative nitrate levels keep below 5ppm consistently with some room for error. This is an example of a system where there is slight excess, but nitrates availability never rise enough for plant-reprogramming to occur.

Unstable dosing example (Tank example 2)

This is an example of a tank that is dosed 3 times a week with high dosage of nitrates. The tank then gets a 50% water change at the end of the week.

This is an example where heavy water column dosing relative to plant up take rates causes nitrate levels in the tank to fluctuate in a large range, which causes significant plant re-programming. This often leads to algae spikes as well as tip stunting for sensitive species. For most planted tanks, a 10ppm nitrate increase/decrease is a significant enough change to stress sensitive plants.

In tanks with unstable nutrient levels, plants may receive enough nutrients to continue to grow and produce good sized new leaves, but there is a greatly increased chance that plant reprogramming stress causes algae to attach to older growth. Reprogramming stress also cause quicker deterioration of older leaves, which are abandoned when their internal machinery does not match the resource levels of the environment.

A better way to maintain high water column nutrient availability in a stable manner is described below.

EI style dosing in a stable manner (Tank example 3)

In this example, we will dose the same amount of nutrients per week as the unstable example (Example 2) above. Plant uptake rate are also kept similar to above. However, the way we divide the dosage is changed significantly. Instead of dosing 9ppm three times a week as above, we now divide the dosage to 3ppm daily. However, on the last day of the week, we do a large water change and do a triple dose to replace nutrients that we removed during the water change. Nutrient buildup = Fertilizer dosed - Plant uptake (cumulatively). The build up rate for Nitrates in this tank is 1ppm daily.

Even though the same amount of fertilizer is dosed across a week for both examples, we have managed to even out the nutrient buildup across the week with this approach.

The key here is that the daily dosage of nutrients should still be close to the daily uptake rate of plants, however, during the water change session, nutrient levels are recalibrated to a higher point rather than trying to hit zero-bound.

The advantage of having plenty of nutrients in the water column is that all plants grow faster. For new aquarists, this provides faster feedback that they are doing things in the right. When one is certain that plants can draw nutrients freely from the water column, it frees up the aquarist to focus on tuning other aspects such as CO2 levels. Newly transplanted plants will adapt faster, and this can give an edge in the cultivation of difficult species.

The main downside to this approach is that many hobbyists may find it hard to find a dosing approach that maintains stable levels. Hobbyist test kits do not provide great accuracy for dialing in dosing rates. It takes a lot of trial and error for most hobbyists to find a dosing regime that works for a particular tank. Many hobbyists spend years tuning their dosing approaches, but still face issues due to the precision required in this approach. 

Most hobbyist test kits offer low accuracy in reading nitrate levels. Higher end kits such as the Hanna Nutrient Analysis Photometer are accurate but cost a lot.

Aquariums with heavy water column dosing also suffer from more severe algae issues when algae is triggered and this is especially so in new setups or when the aquarium does not have enough plant mass. This can be very penalizing to new aquarists who are not used to handling algae.

Algae blooms are much more magnified when triggered when water column nutrients are elevated. A newly set up aquarium can see spectacular diatom blooms when high nutrients are combined with high light. Aquarium pic from Nate B. 

Conclusions

Certain styles are more suited to zero-bound stability systems such as Nature Style aquascapes using easy adaptable plants or Iwagumi aquascapes that use only rooted carpeting plants. The overall stability and slower growth approach of lean water column dosing allows these aquascapes to mature smoothly without drama. The ADA system and its world wide franchises have demonstrated this approach very effectively.