Know your water, know your fish!

For my first proper blog post I thought I would talk about one of the less obvious parameters to consider when choosing fish for your aquarium. Everyone (hopefully) knows about cycling a new aquarium and making sure the filter and water are sufficiently mature to avoid potentially fatal spikes in water chemistry when adding fish, and most people will consider the compatibility of fish with each other, but what some people perhaps don’t think about is the compatibility of their chosen fish with their water – in particular hardness.

I wont’t go into the detailed chemistry of how it works but there are two types of water hardness – general hardness (GH) and carbonate hardness (KH). General hardness is the result of divalent metal ions in the water, principally calcium and magnesium. Carbonate hardness is caused by the quantity of carbonate/bicarbonate ions in the water and is what dictates your waters’ ability to buffer (resist) pH changes – higher KH will give more stable pH.

The hardness of your tap water will be almost entirely dependent on your geographical location – more specifically the geology of your region. A lot of people’s water will be somewhere in the middle of the scale and not present much of a problem (lucky you!), however if like me you live somewhere where the geology is dominated by chalk, you are likely to have very hard water. I won’t bore you with the science (I could, I did a geoscience degree…) but chalk is a type of calcium carbonate, and is the aquifer rock for the majority of the south of England; this results in high quantities of calcium carbonate dissolved into the water – increasing its hardness.

Now what does this have to do with fish? Well for a lot of fish, it isn’t a case of life or death, but is something to bear in mind when selecting/acclimatising fish. As an example I’ll consider the two extremes that spring to mind:

Amazonian fish – many of the fish you see in aquatics retailers will be from the general Amazon/South America region. These areas are largely dominated by very soft, acidic waters. This is due in part to the ancient soils with very little mineral content to leach into the water, and the high vegetation content increases acidity. Rain water is also naturally soft as the mineral ions are not evaporated with the water – high rainfall further softens the rainforest waters. The real extreme case is the blackwaters which have virtually no measurable hardness and very high acidity. For obvious reasons fish taken from these parts of the world will struggle to adapt to the hard waters of southern England.

That isn’t to say that you can’t keep fish from the Amazon and other soft water natives in hard water, I’ve successfully kept many types of tetra, angelfish and ghost catfish to name a few; you just have to be aware, and make allowances. When deciding whether or not to buy fish I know are from soft water there are a few things I bear in mind:

  1. Are these fish currently being kept in a soft water system? Some aquatics retailers near me have dedicated soft water systems set up to overcome the hard water. This is something I cannot provide, and fish coming from this set up will likely struggle to adjust to my hard water. Conversely, if these fish have been bred and raised in hard water they will almost certainly settle in without a problem.
  2. Are these fish notoriously fussy? There are some soft water fish that I personally wouldn’t even consider keeping without a soft water set up – discus and knife fish to name a couple. Others such as angelfish and most tetras will usually do just fine in harder water (provided point 1 has been considered).
  3. How mature are these fish? I have no evidence for this per-se, purely just observation, but in my experience I have had greater success buying young tetras and raising them in hard water, rather than buying adults that may struggle to adjust to harder water after a lifetime in soft water.

Commonly seen soft water natives include: discus, knife fish, angel fish, tetras, various plecos.

Rift valley cichlids – in stark contrast to the conditions of the Amazon, the rift valley lakes of central Africa (Lake Malawi and Lake Tanganyika being the best known) are about as hard as water gets; high mineral content, low levels of vegetation and high evaporation/low rainfall lead to alkaline, very hard waters. Fish from these areas are perhaps less commonly kept due to their generally aggressive, non community nature, and sometimes very large size. I personally kept Tanganyikan cichlids for many years and they thrived in my hard water, breeding prolifically. However they are a challenge, and some do prove very aggressive especially when breeding. Perhaps a more accessible example of hard water fish are certain species of rainbow fish, although this varies by species.

Commonly seen hard water natives: rift valley cichlids, various synodontis catfish.

If you decide that you want to tailor your water towards your chosen species rather than the other way round, there are a few things you can do:

Make water harder – if you wish to make your water harder you can add carbonate rocks (limestone) decor to your aquarium, these will gradually leach minerals into your water increasing its hardness. You could also add a bag of limestone chips to your filter canister to achieve the same effect.

Make water softer – Perhaps more troublesome to achieve/maintain, the most effective way to soften water is to use reverse osmosis (RO) water. RO water has had all the minerals removed by being passed through a semi-permeable membrane, and therefore lacks the minerals that cause hardness. RO water can be bought from your local aquatics centre, but can be pricey to maintain in the long term. That being said, water hardness has a simple relationship – if your water has a GH of 10, and you dilute it 50/50 with RO water, your new water will have a GH of 5, so you won’t be filling you entire tank with RO water (as an aside, RO water lacks almost all minerals and some may need to be added back in as they are required by your aquarium inhabitants). As mentioned previously rainwater is also soft, and can be used to dilute hard water however I would not necessarily recommend using rainwater in this way.

As a piece of general advice, I would just say make sure when introducing any fish to water chemistry that may be acceptable, but different to what the fish is used to, take extra care when acclimatising your new fish; perhaps consider the use of a drip syphon rather than just floating the bag and releasing.

Generally speaking, you can usually keep a nice community with a mix of fish from different parts of the world whatever your natural water chemistry, as long as you are realistic (e.g. not keeping sensitive discus is hard water) and take care with choosing and acclimatising your specimens.

As a summary:

  • research the origin of your fish
  • consider its current environment
  • is this fish likely to adapt? does it require extra care acclimatising?
  • If your heart is set on a delicate species (i.e. discus) consider a dedicated set up with modified water chemistry.

Sorry for rambling on and congratulations if you’ve made it this far, hope you found it interesting!



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