Should you siphon your aquasoil substrate? How to maintain aquasoil substrates?

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 (months/years). Many of the concepts here will also apply to inert substrate tanks. For slow growing tanks, and for people who dismantle their tanks annually, it may not be obvious whether the substrate is well maintained or not. However, substrate problems affect many people - and they affect plant growth and overall tank stability in ways that cannot be compensated for by water parameters/fertiliser management alone.

Organic detritus consists of partially decomposed old leaves and animal waste. While it will release some nutrients into the substrate zone over time, a large accumulation of this waste will interfere with root formation. Delicate plants prefer a cleaner substrate, as counterintuitive as the idea of 'clean substrate' may sound.

Hygrophila sp. Chai is a delicate stem plant; it is very susceptible to algae and you need a clean substrate to grow well-formed specimens.

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 the depletion of nutrients and the buffering capacity of the substrate. Many aquasoils are enriched in ammoniacal nitrogen and this provides a rich source of nitrogen for rooted plants. This is why aquasoil substrates will always grow plants better than inert substrates that are simply enriched with root tabs. 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. Depending on the amount/type of soil used, water change schedules and growth cycles, aquasoils will begin to significantly deplete their nitrogen stores after 6 to 10 months. In soft water tanks their buffering capacity may last a little longer.

Many species, such as Eriocaulon quinquangulare and Blood Vomit, grow faster and more stable with ammonia-rich aquasoils. Many stem species also do better despite having smaller root systems.

The second aspect is that organic waste builds up in the substrate over time. This comes from old decaying roots as well as the decomposition of organic debris (such as old leaves) in the tank environment. This build-up of organic material has a huge impact on fragile, smaller plants and their root systems. Hardier, larger plants tend to be less affected. This organic build-up leads to poor growth, algae and melting of the lower stems. It is not entirely clear whether this is due to overly labile conditions or an overactive decomposer microbial community.

The accumulation of organic detritus is one of the key factors plaguing people with persistent BBA problems.

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.

Organic detritus builds up very quickly in tanks with decaying plants. This is doubly punishing for beginners who are struggling to get their tanks right from the start. Then, as the detritus from the first round of decaying plants builds up in the substrate, it becomes harder to grow the next round of plants, algae problems arise, and so on. People who do not know how to deal with this often end up dismantling the tank and starting again. Such restarts are really not necessary; just removing the organic detritus from the top layer would do the trick.

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.

Clearing build up of organic material

If you are running a very high light tank with fast growth rates or high bio-loading, which usually means higher levels of detritus being produced in an environment where algae are easily triggered, it is important to vacuum the surface of the light substrate every week as part of the water change schedule. We call this Water Change the 2Hr Way. 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.

Here are a few videos to demonstrate this:

Deep cleaning once in a while; this is something that should be done for tanks that have a lot of old roots (due to dense planting) or aquasoil substrates that have been used for years. This can only be done after all the plants have been uprooted, so it is usually done with the replanting cycles for plants. It is the same as above, except that the aim is to remove detritus from deeper layers. 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.

Enriching substrate with root tabs/aquasoil

There are two main ways to replenish depleted aquasoils. The first is to add a new, rich, 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. Our 2hr Aquarist APT Jazz root tabs are designed to be rich in slow release ammoniacial nitrogen for this exact purpose.

Nutrients such as potassium and magnesium are easily absorbed by aquatic plants through their leaves, whereas ammoniacal nitrogen in the substrate is beneficial. 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.

For this purpose, root fertilisers containing ammoniacal nitrogen are much more effective than if the nitrogen source were nitrate-based. Terrestrial slow release fertilisers may also contain trace elements that are too heavy or in the wrong proportions, so if you are using terrestrial slow release fertilisers, stay away from those that contain trace elements.

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