Hard and soft aquarium water – explained

Here’s a scenario. Imagine you’re trapped on a treasure island. The good news is you can keep tropical fish and have unlimited equipment and budget. Everything you want will be provided for the rest of time, but with one caveat, you can only keep hard water or soft water species. For some, this is a quick, easy decision but for the fishkeeping addict, this can be a very real dilemma and for fishkeepers up and down the UK, a real postcode lottery.

What’s the difference?

Hard water is water which has come into contact with calcium and magnesium ions, typically by coming into contact with limestone rock in the ground. Non-fishkeepers see it manifest itself as limescale in the kettle and a scum rather than a lather when they use soap. Hard waters of the UK include nearly all of the south, south-east, east Anglia, east midlands, Lincolnshire and up to Hull in East Yorkshire and accounts for 60% of all UK homes. You’ve only got to watch all the kitchen and bathroom descaler product adverts to see that tackling it is also big business.

Hard water is measured by water companies either in parts per million or milligrammes per litre (which are the same thing,) with 100ppm being classed as slightly hard, but generally upwards of 200ppm (or mg/l) being classed as hard. 300ppm + is very hard.

You can have Temporary Hardness, caused by calcium carbonate (limestone, reacting with carbon dioxide in rain to form soluble calcium Hydrogen Carbonate. It’s temporary because boiling it can remove it, and the hardness salts then leave the solution and deposit themselves typically on our metal surfaces. This is what limescale is, and even stalactites in caves. Permanent Hardness also comes from water percolating through rock, but this time Gypsum (calcium sulphate – not limestone, calcium carbonate.) Calcium sulphate does not precipitate out as Calcium carbonate does, but you still get soap scum instead of a nice bubbly lather like the soft water areas enjoy.

Soft water is essentially what rainwater is, so before hitting any rocks in the ground rainwater is free of any mineral salts as it is formed always from evaporation, and evaporation or distillation never takes any salts with it. When mixed with carbon dioxide in the air, soft rainwater can dissolve limestone causing caves to be formed underground. Soft rainwater takes up calcium from limestone, becomes temporarily hard, and the cycle continues. Soft Water areas of the UK include Scotland, Wales, Cornwall and most of Northern Ireland, but it is always best to check your tap water yourself using test kits as towns and cities can even be split down the middle as to their water supply and source. 

In the aquatic hobby, we can test for KH, which is known as carbonate hardness, temporary hardness and alkalinity. A KH test kit measures for both carbonates and bicarbonates and as bicarbonate buffers KH and keeps it high, bicarbonate of soda is a popular, cheap DIY method of buffering KH in freshwater aquariums. Because those carbonates are temporary, KH can drop, which is the bane of reefkeepers where corals demand a pretty constant, high KH of 7-9 dKH. KH can also drop in freshwater aquariums which causes problems because KH buffers are also good at buffering pH.

If you’re KH drops your pH will also drop, causing first a KH crash, then a pH crash. KH and pH crash is therefore common in soft water areas and is a real problem. This is why even soft water aquariums containing soft water-loving fish should be buffered, and also why you need to add KH buffering minerals to Reverse Osmosis and Deionised water. Because both processes remove mineral buffers producing very soft product water.

The other type of hardness we test for in aquariums is General Hardness or GH. General hardness again tests for calcium and magnesium but not the temporary carbonate (KH) form. Instead, it measures the non-precipitous sulphate form, which drops much less readily. The freshwater fishkeeper needs to have both a KH and GH test kit as they measure two types of hardness, and many a fishkeeper may be surprised to find that they can have both a low KH and high GH at the same time.

The aquariums I used to monitor and maintain in Wigan, Lancashire, where I worked, suffered from just that scenario and any freshwater body ranging from community tanks to planted tanks, to high-end koi ponds, all needed regular KH buffering in order to avoid KH crash and the inevitable pH crash. I find cases of dropsy much more common in water with a little or no KH reading or 0-1dKH. Even with species which naturally occur in soft waters.

So when speaking generally about aquariums and aquarium fish and their habitats we nearly always link high pH with hard water, so water which is both hard and alkaline, and low pH with water which is always soft as well as acidic. But as we found out you can also have low pH and low KH water which has a high GH.  

Is hard water or soft water best? 

High KH will resist KH and pH crash the best so in hard water areas these two scenarios will not manifest themselves very often in either aquatic stores or with their customers unless that is, they are using RO water. The problem with hard water with a high pH however is that ammonia will always be in its toxic form. So soft water is best on your fish in the early days of being a new hobbyist and mistakes being made with filter maturation, but apart from that there really is no best apart what is best for the fish.

In nature, fish have spent millions of years adapting to survive and thrive in either hard or soft freshwaters to the point where, when presented with the wrong hardness they will fail to reproduce at best but at worst, their whole physiology will fail and they will die. Generations of tank breeding can reverse this to an extent like with Angelfish for example, which only naturally occur in very soft, very acidic waters but their now very different aquarium bred mutations will be happy in all but the very hardest of waters and generally show no ill effects whatsoever.

Going with what your tap water is, is the path of least cost and hassle for the hobbyist. By matching softwater fish to soft tapwater and vice versa is the easiest and you won’t need to use expensive buffering or softening aids.

The fun part – fish selection 

The hobby is full of some very notable soft and hard water fish. Here are a selection of popular fish and their habitats but also with a few less common species you may also want to try if you’re feeling more adventurous.

Common types of soft water fish

Amazonian species – be they cichlids, l-number catfish, tetras or piranha, the amazon softwater fish species make up a huge chunk of our hobby, past and present. Due in no small part to the amazonian rainforest and its leaf litter, frequent rainfall and non-lime-based geology, the Amazon is the largest tropical soft water habitat on earth.

Congo species – The Congo river is to Africa what the Amazon is to South America, and is filled with freshwater Characins, catfish, barbs and oddities like elephant noses. Think of a jungle, think of a river flowing through it and you have the Congo and its bizarre, old-world flora and fauna right there.

Peat swamp species – Asia’s threatened peat habitats are home to many popular aquarium fish including chocolate and liquorice gourami, pygmy rasbora and Betta species. Stain your soft water with black leaf and peat tannins for realistic effect. 

Hillstream species – where rainwater falls off non-lime based mountain rocks water moves at force over obstacles instead of dissolving through them. The result is clear, fast-moving soft water habitats with fish representatives in the hobby like hillstream loaches.

Also look out for:

Bee shrimp – these southeast Asian hillstream dwellers come in now very popular red and white but originate from soft water, bamboo edged streams in China.

Other south Americans – This mighty continent contains other great river and lake drainages which don’t meet the mighty amazon, including those in Paraguay, Uruguay and Southern Brazil. Catfish, characins and cichlids still abound here and their species are highly desirable and sought after. Uruguay also contains temperate soft water species including Gymnogeophagus cichlids.

Other Africans – Nigeria is on separate river drainage to Congo but still contains absolute softwater gems including Pelvicachromis, killifish, Jewel cichlids and African snakeheads.

Common types of hard water fish:

Livebearers – Guppies, mollies, swordtails, platies and their relatives have adapted so well to hard water that it’s made them the dominant small fish group in this part of South America.

Tanganyikan cichlids – The hardest waters of the big three African Rift lakes make this diverse and endemic group of cichlids perfect for tanks in the UK’s hardest water areas. Whether it’s big or small, colourful or cryptic, substrate spawning or mouthbrooders you’re after, Lake Tanganyika has it all.

Malawi cichlids – Natural hard water, colour and activity in abundance makes Lake Malawi cichlids deservedly popular, and keeping them for many is a hobby in its own right. 

Central American cichlids – Central America has lots of limestone geology making most of its waters hard as they drain through and spring up from the white porous rock. Be it hard water river or lake, there will be a cichlid living and thriving in it. 

Rainbowfish – these Australian and Indonesian beauties provide much-needed colour and class to those who want hard water fish without involving cichlids.  

Also look out for:

Rice fish, Oryzias spp. Rice fish are tiny Asian fish which inhabit hard freshwaters and some brackish waters and are known for their unusual breeding method whereby eggs cluster to the female’s vents before being dislodged and stuck to vegetation.

Goodieds – these alternatives to standard livebearers overcome many extreme and isolated hard water habitats in the wild, only to find themselves under threat from habitat destruction and introduced species. One for collectors and conservationists.

Lake Inle fish – Danio erythromicron, Yunnanilius brevis, Sawbwa resplendens and Inlecypris auropurpurea all go to together to make this lovely and unique Asian community lake biotope.

Sulawesi fish and shrimp – tinted blue from all the dissolved minerals, the lakes of Sulawesi (formerly Celebes,) contain some very sought after, marine reef looking freshwater shrimp which need hard water, and the lovely Celebes rainbowfish, Marosatherina ladigesi too.

Crater lake and soda lake cichlids – unique habitats in Central America and Africa boil up through volcanic rock and can be extreme in temperature and causticness. Naturally some all-conquering but now endemic cichlid species make this habitat their home.

Does hardness affect ammonia toxicity?

Ammonia toxicity is affected by two things – pH and temperature. The higher the pH, the more toxic ammonia becomes. The high the water temperature also, the more toxic ammonia becomes. So hot, hard water is the most toxic and cold, soft water the least. 

KH can affect pH however and CO2 can lower pH, so indirectly, yes, KH can affect ammonia toxicity because of its effect on pH. In soft water with low pH, ammonia is much less toxic because it becomes un-ionised NH4 instead of NH3.

*Cardinal tetras photographed at Pier Aquatics, Wigan, UK


Jeremy Gay

Author of three fishkeeping books and lifelong fishkeeper. Experience includes editor of Practical Fishkeeping magazine, editor of Pet Product Marketing magazine, multi award- winning livestock manager and aquatic store manager.