Do substrates/aquasoils needs maintaining?

Aquasoils are now standard in many planted tank setups, but there is very little, if any, information available on how to maintain aquasoil substrates over the long term. Many aquarists that do only short term setups for photographs or social media content will treat aquasoil as being maintenance free. However, proper maintenance of the aquasoil substrate has a large impact on algae, and health of plants in the long term.

A well maintained aquasoil tank does not need to be completely torn down for many years. There are two main things that happen over a longer time horizon that are not immediately apparent to most aquarists. The first is that organic debris accumulates and the second is that the nutrient content of the soil depletes. 

Aquasoil maintenance is key in higher light tanks and for tanks attempting to propagate more difficult plant species. It is also important for aquarists that want to reuse the same aquasoil substrate over the longer term horizon.

1. Clearing organic detritus

Organic detritus consists of partially decomposed old leaves and animal waste. While detritus will release some nutrients into the substrate zone over time, a large accumulation of this waste will interfere with root formation for picky plants. Delicate plants prefer a cleaner substrate, as counterintuitive as the idea of 'clean substrate' may sound. It is not entirely clear whether this is due to overly labile conditions or an overactive decomposer microbial community that causes problems with root formation - what is clear is that allowing detritus to accumulate to high levels produces both poorer plant growth as well as algae. 

Example of a tank with a lot of organic detritus. Delicate plants will not grow well here, regardless of the tank parameters, and this is an invitation to algae in the long run.

This outcome is counter to the expectations of many aquarists. Low tech aficionados romanticize the idea of organic detritus contributing nutrients to the substrate layer over time, similar to a farmer fertilizing his field with manure. In reality, fish waste is not an effective way to fertilize plants in the aquarium. It does not have a complete nutrient profile and neither does it integrate well into the substrate layer - but it does trigger algae easily in higher light setups. 

In lower light systems where algae growth is light limited, it is not as penalizing to skip cleaning the substrate surface. However, in higher light tanks (150 umols of PAR and above), it makes a critical difference in algae outcomes. The accumulation of organic detritus is one of the key factors plaguing people with persistent Black beard algae (BBA) problems.

Removing organic debris on the substrate surface can be done together with the weekly water change.  For slower growing tanks with low bio-loads, this can be done less frequently, perhaps once every 2 or 3 weeks. This is done by hovering a siphon over the substrate surface while using a turkey baster to scrape up surface debris. For weekly cleaning you should aim to disturb no more than the top 1cm or so of aquasoil. We call this Water Change the 2Hr Way.

Here are a few videos to demonstrate this:

Deep cleaning of aquasoil should be in longer intervals (6 months to 1 year). This is something that should be done for tanks that have a lot of old roots and where fine organic detritus has settled down to the deeper layers of the substrate over time. In regular substrate maintenance we do not want to disturb the ecology of the deeper zones of the substrate. However, fine organic detritus accumulate over the very long time horizon and needs to be cleared for the substrate to work optimally.

This can only be done after all the plants have been uprooted, so it is usually done when there is a major replanting of the aquarium. This is also done when the aquarist observes that plant growth has become anemic due to substrate zone compaction and that plants do not root as well as before.

There is no particularly delicate technique to this; you can move the aquasoil around with a small spade or your fingers, sucking up smaller particles and detritus; this goes hand in hand with the manual removal of old root parts of old growth that are trapped in the soil. This would usually be accompanied by a large water change, as stirring up deep layers of substrate usually releases ammonia and labile organic matter into the water column.

Checking plant roots during replanting cycles can be an indication of substrate health. Blackened and/or mushy roots can indicate substrates that have been left unmaintained for too long.

Rotala tulunadensis, Rotala florida, and Rotala sunset are all sensitive to substrate quality and cleanliness.

2. Enriching substrate with ammoniacal nitrogen

Many aquasoils are enriched in ammoniacal nitrogen and this provides a rich source of nitrogen for rooted plants. It is this source of nitrogen that makes plants grown in aquasoil tank larger and more robust compared to plants grown in just inert substrate. Good quality aquasoils will begin to significantly deplete their nitrogen stores after 6 to 12 months. 

Ammonicial nitrogen produces fuller crowns in Ludwigia pantanal (above) and Rotala macrandra (below).

Laboratory analysis of aged aquasoil samples from Tom Barr's tanks (Barr Report Vol5, Issue 1) shows that although ammonium levels are depleted over time, the P, K and Fe levels of aged soils were actually higher than in new aquasoils. This may be largely due to the high water column nutrient dosage in this particular tank, demonstrating that water column nutrients do find their way into the substrate over time. The key nutrient to be replaced as aquasoil ages is ammoniacal nitrogen.

There are two main ways to replenish depleted aquasoils. The first is to add ammonia-rich aquasoil from time to time (every 3 to 6 months is a good rate). The other method is to use nitrogen rich root tabs. Adding new aquasoil can be done during plant replanting cycles. A portion of old aquasoil can be removed with the water siphon, or with a spoon, and new aquasoil added and mixed in with old aquasoil. This method may be cheaper than adding root tabs in the long run.

Adding 10-20% new ammonia rich aquasoil in an area makes a significant impact on plant growth. The tank above is grown in using 1 year old APT Feast, mixed in with 15% new aquasoil.

Ammoniacal nitrogen gives denser, fuller growth to stem plants and gives an edge in propagating picky plant species such as Eriocaulons and Syngonanthus species.

Our 2hr Aquarist APT Jazz root tabs are designed to be rich in slow release ammoniacial nitrogen for this exact purpose. If dosed into the water column instead, ammonia would quickly oxidise to nitrates in the water column and may also trigger algae growth. Nitrates, on the other hand, don't readily bind to soil - so having them in root tabs just means that they will slowly leach out and contribute to NO3 levels in the water column.

This tank in the 2hr Aquarist gallery has been maintained with the same Aquasoil substrate for over a year. The stem plants have been trimmed and replanted for many cycles without changing the substrate. The consistent maintenance and enrichment of the substrate makes it look as if each new growing cycle has been planted in fresh Aquasoil.

Head here to read on more substrate topics

Head here to read more on water column vs substrate fertilization