Guest Blog: Hydrography and the soft BWBS aquarium by Ian Davis

Editors's Note: If there is one thing that I find fascinating about the world of blackwater, botanical-style aquariums, it's that you never stop learning. You never stop questioning...and with our crowd, you never stop applying what you've learned! One of my fave "BWBS" aquarium philosophers is Ian Davis. He has a keen understanding of the challenges, practices, and joys of these unique aquariums. 

We often talk about the use of "soft, acid water", or water of "little to no carbonate hardness" in the context of our work. It sounds all well and good- but what exactly do we mean? In this guest blog, Ian provides a very nice writeup on the ins-and-outs soft water, and how it applies to our work! We think you'll enjoy his concise, informative style and great information! 


Ok, so for many of you this will be a new term in botanical-style, blackwater aquariums. So let's start at the beginning.  This describes the overall physical characteristics of the water in our aquariums.....  

So our idealised soft water environment is one that contains low levels of dissolved minerals.  It is considered that water that has less than 0-3.37 degrees general hardness to be "soft water" and between 3.38-6.74 degrees general hardness "moderately soft water" (this is just my take on this based on my research so others of you may have different values).  It is also quite common for soft water to be described in terms of calcium carbonate concentration, in which case soft water has less than 60 mg/l calcium carbonate and moderately soft water between 61 and 120 mg/l calcium carbonate. 

Soft water exists wherever water is flowing across terrain that is poor in soluble minerals (podzol substrates and soils amongst others).  The rivers of the upper Amazon are predominantly of this type and are what we term "black water" i.e. that which we are all trying to recreate.  Because so many of the ornamental fish sold in the trade come from these areas, many of us assume that they need or prefer soft water conditions in the aquarium. Up to a point this is indeed the case, but creating and maintaining a soft water aquarium places challenges on us fish keepers.

1. Soft water exhibits a stronger tendency towards pH instability than hard water.  All aquariums become more acidic over time, but in a soft water aquarium this can occur faster.  Since few fish will tolerate rapid changes in pH, frequent pH tests and the use of chemical buffers therefore become a necessity in maintaining a soft water environment.

2.  Filter bacteria work best in hard, alkaline water conditions.  In very soft and acidic water, filter bacteria work less efficiently and activity decreases as the pH and hardness of the water decreases, forcing us to use less efficient methods of filtration or chemical media within our filtration systems to compensate.

3.  Producing soft water from hard, tap water can be expensive.  Reverse-osmosis and/or deionisation will accomplish this but this comes at a price.  Reverse Osmosis (RO) is a water treatment process that removes contaminants from water by using pressure to force water molecules through a semipermeable membrane. During this process, the contaminants are filtered out leaving clean, purified water.  Deionisation (DI) is a chemical process that uses specially manufactured ion-exchange resins, which exchange hydrogen and hydroxide ions for dissolved minerals, and then recombine to form water.  On the other hand collecting rainwater is a zero cost alternative, but it is not without its own drawbacks.

So why bother with soft water aquaria at all?

While many Tetras, Barbs, Gouramis and Corys will do well in harder water many Angelfish, South American dwarf cichlids etc will struggle to adapt to these conditions and while they will survive in these conditions they are unlikely to fully adapt even after multiple generations in captivity.

In the first place, soft water may be critical to breeding these fish. While tetras and South American dwarf cichlids will live in harder waters, they will not readily spawn.  Sometimes they will deposit their eggs but because of the improper water chemistry, the eggs will not develop. In other instances they simply won't show breeding behaviours at all.  Now if you are only interested in keeping a community tank of these species then it is unlikely to be of concern, however by doing so we are limiting our fishes natural instincts and behaviours by providing them with conditions to live in that do not best replicate their natural environment.

If you live in a soft water area you may have water of appropriate pH and hardness for a soft water aquarium. Maintenance of the aquarium will be relatively straightforward because you can perform large, regular water changes with minimal issue and thereby minimise any problems with water quality and potential pH swings.  For aquarists in hard water areas things become more complicated.  Soft water needs to be made up by mixing a small proportion of hard water with a larger proportion of pure (RO/DI) water. This will dilute the hardness in the tap water resulting in something with a pH and hardness that will suit our fish.

RO water is convenient because you can produce pure water as required. RO filters come in different sizes scaled to the demands of the user.  However, as already discussed, the filter cartridges and membranes of RO/DI filters are moderately expensive and need to be replaced at regular intervals.

For those of us with more limited resources, rainwater can be collected and stored for use but it is important to clean the water before use.

Setting up a rainwater butt to collect water from the gutters on your home is easy to do and the equipment is inexpensive.  Obviously the rain itself costs nothing however the downside to rainwater use is that it depends on regular rainfall, something not all of us can rely on, particularly those in hotter regions of the world than the north of England.  It is also important to store a certain amount of water to allow for water changes during dry spells.  Simple filtration through carbon will remove most soluble pollutants however detritus on the roof and in the gutters is more of a problem as this can contain insoluble and inorganic particulates from your roof.  It is therefore important to run the collected rain water through a filter to remove the accumulated detritus from the water before adding to the tank.  This can be done using simple filter papers or more sophisticated membranes but the key here is to ensure that all debris is removed at this stage.

Because soft water is poor in minerals, various trace elements supplements are produced for aquarists keeping soft water aquariums.  These supposedly help the health of the fish, and are simply added to water at regular water changes.  Whether or not they are useful will depend on how soft the water is that you are using.  Alternatively botanicals can be used to provide trace elements to the water as well as providing that all important "tint".  

As has been discussed here on many occasions, just adding botanicals to a hard water aquarium wont turn it into a soft water aquarium, this merely changes the colour of the water by releasing tannic acids etc and does not directly influence the softness.


Other methods of softening water

Peat can be used to soften and acidify water however this has largely fallen out of favour, particular since the advent of RO/DI filters. Peat acts in a similar manner to ion exchange resins by removing minerals from the water and replacing them with organic (tannic) acids.  Besides softening the water, peat turns the water a dark brown in much the same way as our favourite botanicals do when added to the tank.

The main problem with peat is that it is unpredictable and is becoming frowned upon for environmental reasons.  Adding peat directly to the aquarium (for example inside a canister filter or a compartment within you sump) will soften and acidify water over time, but the rate at which it operates will depend on the original hardness of the water.  Water that is very hard may not change very much at all, while water with relatively low hardness can become acidic quite rapidly.  This is by no means an exact science and will fluctuate in both the degree of water softening and the time scale with which this occurs.


The soft water aquarium presents its pro's and con's for us fish keepers . On the plus side, there's no doubt that some of the real jewels of the hobby, including Apistogramma,  Rasboras, Tetras etc never look better than when kept in soft, acidic water stained the colour of tea.  

Under such conditions these fish can be seen in their full glory whereas when kept in hard water their colours are often more subdued.  Although the soft water aquarium is not straight forward to set up or maintain the benefits of seeing your fish behaving as natural as possible far out ways the work and with a little effort we can create a tank with vibrant, happy occupants who will thrive to their full potential.

Ian Davis


Scott Fellman
Scott Fellman


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