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Stocking and running a soft water aquarium

29 January 2010

With Maidenhead Aquatics at Truro joining the TFF family, it’s a good time to think about stocking and running freshwater aquaria if you live in a soft water part of the UK. Aquarists in Cornwall are provided with some of the softest water in England, and while that’s good in some ways, it can be challenging in others. In this article we’ll be looking at the pros and cons of soft water aquaria, and what sorts of fishes would be best suited to such an aquarium.

Maidenhead Aquatics at Truro is on the Quenchwell Rd just north of Carnon Downs, about 3.5 miles south of Truro city centre. The tropical fish shop is in the Carnon Downs Garden centre, where there is plenty of parking space. The nearest railway station is Perranwell, 1.7 miles away. From the station, walk northeast along Station Road and into Old Carnon Hill into the village, then onwards through Forth Coth, and then left into Quenchwell Road. The garden centre is on the outskirts of the village.

Water chemistry

Soft water is by definition mineral poor, but a key issue for aquarists is the low level of carbonate hardness, typically less than 2 degrees KH. Without carbonate hardness, there is little in the water to buffer against pH changes. So while pure water has a pH of 7, as soon carbon dioxide and nitrate are dissolved into it, these chemicals form acids, in these cases carbonic acid and nitric acid. This causes the pH to drop. In a tank with moderate to high hardness, the carbonate hardness reacts with these acids, cancelling out the chemical reaction, so there’s no pH change. In a soft water tank with little to no carbonate hardness that can’t happen, and between water changes there is a severe risk of drastic drops in pH.

To prevent pH changes, it is important not to overstock soft water aquaria. The fewer fish, and less nitrate and phosphate there will be in the water, and that means less nitric acid and phosphoric acid to trigger pH changes. Equally important is to keep a regular schedule of water changes. It is better to do lots of small water changes than one big one, so that small pH changes are corrected quickly. Great care should be taken with carbon dioxide generators used to promote plant growth. Too much carbon dioxide and the pH can plummet.

Always use a pH test kit on a regular basis to make sure that the pH of your soft water aquarium does not vary too greatly between water changes. Commercial pH buffers are widely sold and very useful for stabilising pH at a predetermined point. In most cases pH 6.5 or pH 7 buffers will be most useful. Lower pH value buffers are available, but note that the lower the pH, the less effective biological filtration will be.


The bacteria that perform biological filtration work best in hard water at a pH of 7.5 to 8. Below pH 7 they work more slowly, and below pH 6 may not work at all. Aquarists keeping soft water aquaria should therefore avoid very low pH values by using appropriate buffers.

In some cases soft water fish need very low pH values to do well. The classic examples are things like Pikeheads, Ram Cichlids, and Chocolate Gouramis. These fish naturally come from waters where the pH may be 5.5 or even less, and above pH 6.5 tend to be sickly. Seemingly they cannot resist the opportunistic bacteria present at such water conditions. If you need to use very soft, very acidic water conditions, biological filtration may not be an option. Zeolite can be used instead to remove ammonia directly. This will need to be replaced on a regular basis, typically weekly. Up to a point it can be recharged and reused; check the manufacturer’s instructions for details.


Gouramis are a particularly good choice for soft water aquaria. In stock at Maidenhead Aquatics at Truro are some of the most popular species as well as some less unusual species. One of the less often seen species is the Sparkling Gourami (Trichopsis pumila), a very small (to 4 cm) gourami noted for its brilliant blue, pink and purple colouration. Males and females are very similar, and in common with other Trichopsis species, these fish sometimes make croaking sounds. Sparkling Gouramis need to be kept in a peaceful aquaria with lots of surface vegetation; floating plants are particularly appreciated, and given this sort of cover, these fish will swim about in view all the time. They are easily bullied, so do not keep with larger tankmates.

The Honey Gourami (Colisa chuna) is a gourami widely sold in the UK but often reported as sickly and difficult to maintain. One problem with this gourami is that it doesn’t thrive in hard, basic water. In a soft water aquarium it will do much better, and makes a good companion for peaceful community fish like rasboras and tetras. It is very shy, so should only be kept in tanks with plenty of plants.

An even more challenging species is Vaillant’s Chocolate Gourami (Sphaerichthys vaillanti). These are very sensitive and delicate fish, and should always be kept in a single species aquarium. They need very soft water to do well, so the soft Cornish water will be ideal. They also need a strongly acidic pH, between 5.5 to 6.5, and that means biological filtration may not work particularly well. Certainly the tank should be lightly stocked, and if necessary zeolite used to remove ammonia. Water temperature should be between 28-30 degrees C. Vaillant’s Chocolate Gourami is one of the mouthbrooding gouramis, and this is what makes them appeal to advanced aquarists. They happen to be very pretty as well, but that’s a bonus: don’t plan on keeping them in a community tank!

Tetras and other characins

Most tetras and characins prefer soft water, so fishkeepers in soft water areas can pick and choose almost anything they want. About the only commonly traded exception is the Blind Cave Tetra (Astyanax mexicanus), a Central American species that prefers moderately hard, neutral to slightly basic water.

The Glowlight Tetra (Hemigrammus erythrozonus) is a classic soft water species for dark, shady tanks with plenty of plants. In small groups this species tends to be nervous and shy, but keep twenty and they’re a revelation! The glowing orange band running through their transparent bodies really comes alive when they’re schooling together nicely, and adding a little blackwater extract to the water will further improve the effect.

Another excellent little tetra is the Lemon Tetra (Hyphessobrycon pulchripinnis). While adaptable and easy to keep, in hard water this fish rarely displays its best colours, tending to look a bit washed out. But in a soft water aquarium this fish develops a lovely lemon yellow body colouration, highlighted with a bold red marking above the eye and further black, white and yellow markings on the fins. As with any other tetra, keep in a large group for best results.

There are three fish sold as Emperor Tetras in the trade, the true Emperor Tetra (Nematobrycon palmeri), the Black Emperor Tetra (Nematobrycon palmeri), and the Blue Emperor (Inpaichthys kerri). As you’d expect given their names, these are very beautiful fish. They aren’t especially difficult to keep, though they won’t do well in hard water. In a soft water aquarium they should be kept in a group of six or more specimens, but given space, because the males are somewhat territorial. The true Emperor Tetra is essentially iridescent purple with a black band along the middle of the flank and bright blue or green eyes. The Black Emperor Tetra is similar but the black patch on the flanks is more extensive. The Blue Emperor is more luminous blue than purple, and tends to be less hardy than either the Emperor Tetra or the Black Emperor Tetra.


Rasboras invariably do better in soft water than hard, so they’re an obvious choice for the soft water community tank. The Harlequin Rasbora (Trigonostigma heteromorpha) and the Lamb-chop Rasbora (Trigonostigma espei) are the two most popular species. The two species are very similar, essentially coppery with a purple-black L-shaped marking on the flanks, the shape of this marking is slightly different between the two species. They have a reputation for being delicate, but in soft water tanks are generally very easy to maintain. Slightly shady conditions and a profusion of plants will bring out the best of their colours.

The Least Rasbora (Boraras urophthalmoides) is typical of the smaller species that make up the genus Boraras. It is a slender, nicely coloured species that only gets to about 3 cm in length. Its body is semi-transparent, with an orange and blue-black band running along the flank. The unpaired fins are further marked with patches of orange and black. It is a lovely fish for peaceful community tanks that have plenty of plants, but given its tiny size, it shouldn’t be kept with anything substantially larger. Good companions would be things like kuhli loaches and Sparkling Gouramis.


Numerous South American cichlids do well in soft water, but one of the classic species that only ever does well in soft water is Lyre-finned Checkerboard Cichlid (Dicrossus filamentosa formerly Crenicara filamentosa). This small, to 6 cm, slender cichlid is marked with square black blotches across its silvery body, and males especially develop very long tail fin extensions. In hard water tanks it never does well, but in a soft water community tank it makes an interesting companion for small midwater tetras and rasboras. Flake is accepted, but live and wet-frozen foods are preferred, and probably essential for long-term success.

Discus (Symphysodon spp.) are of course the classic soft water cichlids. In general these cichlids are best kept in their own aquarium because they need quite warm water, around 28 degrees C. This will be stressful to most community fish, though Cardinal Tetras and Corydoras sterbai can make good tankmates. Discus are nervous when kept singly, and should either be kept as a matched pair or else in a group of at least six specimens. Good water quality as absolutely paramount to success with Discus, and a large aquarium with a good filtration system will be essential. Given these things, tank-bred Discus aren’t especially delicate, and there’s a huge variety of artificial breeds available to choose from.


Maidenhead Aquatics at Truro maintains a good stock of oddballs that will appeal to advanced fishkeepers. Some are fairly easy to keep, others much less so, and research is crucial.

One of easier to keep oddballs is the Dwarf Puffer (Carinotetraodon travancoricus). This little pufferfish is best kept amongst its own kind, but since they are territorial, allow about 10-15 litres per specimen. Like all pufferfish, Dwarf Puffers are sensitive to poor water quality and shouldn’t be kept in tanks smaller than 35 litres capacity. Otherwise they are not demanding, and although live foods including snails are preferred, they take wet-frozen bloodworms and other small invertebrates very readily. Dwarf Puffers are fin-nippers and quite aggressive despite their size, and cannot be kept in community tanks.

At the other end of the difficulty range is the Malayan Freshwater Pipefish (probably Doryichthys boaja). Like all pipefish, the main difficult here is feeding them. Pipefish, like their cousins the seahorses, will only take live foods unless they’ve been trained to do otherwise. Certainly wild-caught specimens will have little interest in wet-frozen foods and no interest at all in dried foods. Daphnia, brine shrimps and insect larvae will be required. Once settled, they may be weaned onto wet-frozen foods, but unless the retailer can demonstrate them feeding this way in front of you, don’t take it for granted. Only buy them if you can provide suitable live foods, for example from a garden pond.

The Armoured Stickleback (Indostomus paradoxus) is related to the pipefish and seahorses on the one hand, and sticklebacks on the other. It is a very small fish, about 3 cm long, and looks like a stickleback in basic shape, but with the slow, deliberate habits of a pipefish. Like pipefish it prefers live foods, primarily worms, insect larvae and other tiny invertebrates. It’s a very rare species in the aquarium trade, but kept in a single-species planted aquarium can do very well, given sufficient food.

The Red-stripe Halfbeak (Hemirhamphodon kapuasensis) is not quite so difficult to maintain. Unlike the more adaptable Celebes and Wrestling Halfbeaks, this species does need soft water to do well. It is a small (to 6 cm) fish and very slender, and does like to jump. It should be kept in a single-species aquarium with lots of floating plants. Although considered a livebearer, and certainly performing internal fertilisation, this species actually lays very large eggs on floating plants. Once these hatch, large fry emerge that are fairly easy to rear.

What not to keep

Some fish will not adapt to soft water, and shouldn’t be kept in a soft water aquarium. These include Central American livebearers, Central American cichlids, Lake Malawi cichlids, Lake Tanganyikan cichlids, and most of the Eurasian carps, including Goldfish. Brackish water fish will also not do well in soft water. If you want to keep any of these fish, it’s important to adjust the water chemistry accordingly. Rift Valley cichlid salts are useful for this, being cheap and easy to use. Marine salt mix will need to be used for tanks with brackish water species, and can also be used for salt-tolerant livebearers such as Guppies and Mollies.

Invertebrates are a mixed bag in soft water systems. Snails generally prefer at least moderately hard water, and in very soft water their shells become pitted. Although not life-threatening, this does make them look less attractive. Shrimps can tolerate soft water reasonably well, but it’s a good idea to provide them with foods rich in calcium so that they can moult and grow properly; wet-frozen krill, mysis and brine shrimp should do the job nicely.