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 to avoid algae 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.

Good substrate maintenance allows planted tanks to grow well long term. The aquarium above has substrate that is 1.5 years old.

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 and give rise to 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 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. 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 substrate and removal of old root systems

Deep cleaning of aquasoil should be in longer intervals, around 6 months to 1 year+ depending on whether there is compaction in the substrate zone. Dense plant growth combined with relatively thin substrate layers is why substrate zones in aquariums get compacted easily. Old roots and fine organic detritus accumulate over the very long time horizon and needs to be cleared for plants to continue to be able to root well. This is similar to repotting terrestrial plants that have outgrown their pots. 

Plants need to be uprooted and old roots removed. Very fine organic debris is also siphoned away to improve the pore spacing for aquasoil. Siphoning would be done as the plants are uprooted.

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 4 to 6 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. One method is to use nitrogen rich root tabs. The other method is to add ammonia-rich aquasoil from time to time. A good rate is adding 1% of new substrate per month. So if you have 30kg of aquasoil in an aquarium, adding around 300 grams per month will work well.  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.

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

If you do not want to use the add new aquasoil method, our 2hr Aquarist APT Jazz root tabs are designed to be rich in slow release ammoniacial nitrogen for this exact purpose. 

Head here to read on more substrate topics

Head here to read more on water column vs substrate fertilization