Freshwater Biotope Aquariums: Match Fish to Your Tap

Quick answer

Start with your tap, not with a river. Measure GH, KH and pH first, then choose a habitat your water can hold without a fight.
Biotope-inspired is the achievable version. A strict replica - one locality, documented, nothing out of place - is a competition discipline. The home version keeps the behaviour and the conditions without the purity test.
Tannins are a method, not a filter. Leaf litter and wood release tannins and humic substances, and drop pH only where KH is already low. They do not soften hard water.
Hard tap is not a dead end - West African and hillstream setups suit it well. Soft-water communities need RO mixed with tap, and never softened tap water.
Captive-bred first, with one well-known exception worth understanding.

What a biotope is, and what it is not

Strictly, a biotope aquarium reproduces one specific habitat: the fish, the water chemistry, the substrate, the wood, the leaf litter and the plants all drawn from a single stretch of a single river. Judged biotope competitions expect that level of rigour, including documentation of the locality being represented.

That is not what most people mean, and it is not what this series is about. The useful home version is biotope-inspired: a tank where everything comes from broadly the same kind of place, so the fish want the same temperature, the same hardness, the same light level and the same layout. You are allowed a plant from the wrong continent. You are not allowed to put a Rift Valley cichlid with a cardinal tetra and call it a river.

The point is not purity. The point is that a tank built around one set of conditions is dramatically easier to run than a tank built around a compromise between four. Everything in it wants the same water, so you can actually give it to them.

Read your tap before you pick a river

Three numbers decide which biotopes are realistic for you:

GH (general hardness) - dissolved calcium and magnesium. This is the one that matters most to fish physiology, breeding and shrimp moulting.
KH (carbonate hardness) - the buffering capacity that resists pH change. High KH means your pH will not move, no matter how many leaves you add.
pH - the consequence of the other two more often than it is a cause of anything.

Your water company publishes a hardness figure by postcode, which is a good starting point, but test it yourself: pipework, storage tanks and household plumbing all change what reaches the tank. Our GH and KH guide covers how to read the numbers and water parameters covers the rest of the panel.

Broadly, in the UK: the south and east tend to be hard, with high GH and KH and a pH around 7.5-8. Much of Scotland, Wales, the south-west and parts of the north-west are soft. Neither is a problem. They are just different starting points, and they point at different rivers.

Softened tap water is not soft water

A domestic water softener works by ion exchange - it swaps calcium and magnesium for sodium. GH falls, KH does not, and the sodium content rises substantially. That is not the soft, mineral-poor, low-buffer water a blackwater fish evolved in, and it is not safe to use as a substitute. If your house has a softener, take water from the unsoftened kitchen tap, or use RO.

If you have hard tap and want genuinely soft-water fish, reverse osmosis water blended back with tap is the only reliable route - you mix to a target GH rather than trying to strip it out chemically. Chasing a pH number with bottled adjusters is the classic beginner trap and it produces unstable water, which is worse for fish than the wrong-but-steady water you started with. See pH problems for why.

Three starter pillars

These are the first three instalments in this series, chosen because between them they cover almost every British tap water situation:

Pillar	Water it wants	On hard tap?	Typical stock
SE Asian blackwater nano	Soft, acidic, low KH, dim	Needs RO blending	Boraras rasboras, shrimp, snails
Amazon-inspired soft community	Soft to moderate, slightly acidic	Workable with care	Cardinal tetras, corydoras, a dwarf cichlid pair
West African or Asian hillstream	Moderate to hard; hillstream also cool and fast	Yes, straight from the tap	Kribensis and Anubias, or hillstream loaches

If you are on hard water and impatient, start with the third one. It is the only pillar here that asks nothing of your tap.

Botanicals and tannins: functional, not decorative

Leaf litter is the part of biotope keeping that looks like an aesthetic choice and is actually a functional one. As leaves and wood break down they release tannins and humic substances, which tint the water, provide a surface for biofilm - the first food of shrimplets and fry - and are widely reported to have a mild antifungal and antibacterial effect.

What they will not do is soften your water. Tannins can pull pH down, but only when KH is low enough to let it move. On hard, well-buffered tap water you will get tea-coloured water at the same pH you started with. That is worth knowing before you buy a kilo of catappa leaves expecting a chemistry change.

In practice: Indian almond (catappa) leaves, alder cones, and oak or beech leaves collected somewhere unsprayed and given a boil first. Add gradually rather than all at once, and replace them as they break down - a few weeks to a couple of months depending on the leaf. Our tannins guide and wood and tannins go into the detail.

Two ways botanicals bite back

A heavy botanical load decomposing in a small tank consumes oxygen, which is how a blackwater nano ends up with fish at the surface. And in a tank with almost no KH, adding a lot of acidic material at once can drop pH sharply overnight. Add in stages, keep some carbonate buffer rather than driving KH to zero, and keep the surface moving.

Captive-bred first, with one honest exception

For almost every species in this series, captive-bred stock is the better choice: it is hardier, already adapted to aquarium conditions and aquarium food, less likely to arrive carrying parasites, and it takes no pressure off wild populations. That covers kribensis, most Boraras on sale, dwarf cichlids, corydoras and every shrimp you will buy.

The exception worth understanding is the Rio Negro ornamental fishery. Cardinal tetras are still largely wild-caught, from a long-running, seasonally regulated fishery that provides income to riverside communities - and that income is one of the arguments against clearing the forest those communities live in. "Wild-caught" and "unsustainable" are not automatically the same sentence, and it is worth knowing which one you are actually looking at. The trade-offs, and how to tell, are in wild-caught versus captive-bred.

Whatever you buy, wild-caught fish need more care on arrival: slower acclimatisation, a genuine quarantine period, and water closer to their source parameters than a captive-bred fish would demand.

Building the stock list without guessing

A workable method for any of the pillars above:

Pick the region first, using your tap numbers as the constraint.
Choose one shoal - the dithers that occupy midwater and make everything else feel safe.
Choose one centrepiece or pair - usually the cichlid or the larger characin.
Choose one bottom group from the same region, if the habitat has one.
Check it. Run the list through the stocking calculator for bioload, group minimums and pairwise compatibility, and read the individual entries in the species database for temperature and parameter ranges.

App-aquatic's origin and biotope analysis does the same job from the other direction: it maps where the fish already in your tank come from, which is a quick way to find out that your "Amazon" tank contains two Asian species. The stocking calculator guide explains how the bioload and compatibility engine reaches its verdicts.

The series roadmap

Three instalments follow this hub, each a full setup guide with parameters, hardscape, planting, a stock shortlist and a maintenance routine:

1. South East Asian blackwater nano. A 30-45 L peat-swamp-inspired tank: chilli rasboras or another Boraras, deep leaf litter, catappa, spiderwood, dim light, minimal planting beyond moss and floating cover. The soft-water path for people on hard tap, done with RO blending rather than chemicals. Shrimp and snails where they fit.
2. Amazon-inspired soft community. Sand, wood, dim light and leaf litter, with cardinal tetras, corydoras and a dwarf cichlid pair. Includes the honest version of the UK hard-tap workaround: which of these fish will genuinely live and colour up in harder water, and which will only breed in soft.
3. West African stream, or Asian hillstream. Two routes for hard-water keepers. The West African version pairs kribensis with Anubias and wood over sand. The hillstream version is cooler and much faster: high flow, high oxygen, smooth stone, biofilm grazers - and a chance to correct the two persistent myths that hillstream loaches are tropical and that they need a planted tank.

The mistakes worth avoiding up front

Chasing a pH number with bottled adjusters. Stable and slightly wrong beats unstable and briefly right, every time.
Calling a tank a biotope because everything is brown. Tannins are a habitat feature, not a passport.
Mixing continents and hoping. If two species want different hardness and different temperature, the tank is a compromise both of them lose.
Adding wild-caught fish to a young tank. They are the least tolerant fish you can buy of an immature filter - finish the cycle and let the tank mature first.
Forgetting the fish have to eat. Leaf litter feeds biofilm grazers, not rasboras. A biotope still needs a feeding plan.

If a soft-water tank does crash - a KH of zero, a pH drop overnight, fish suddenly in trouble - work through Tank SOS rather than guessing, and read suddenly losing fish for the wider differential.

Log your tap GH, KH and pH first, then build the list in the calculator and simulate the additions before you order anything.

Build the stock list

Common questions

What is a biotope aquarium?

A tank that recreates a particular natural habitat - the fish, water chemistry, substrate, hardscape and plants all drawn from the same kind of place. Strict biotopes replicate a single documented locality. Biotope-inspired tanks, which is what most home keepers build, keep the conditions and the behaviour without insisting every item comes from one river.

Do I need RO water for a biotope tank?

Only if the habitat you have chosen is genuinely soft-water and your tap is hard. West African stream and Asian hillstream setups run happily on hard British tap. Blackwater tanks with wild-type soft-water fish do need RO blended back to a target hardness, because you cannot reliably strip minerals out of tap water any other way.

Do catappa leaves lower pH?

Only when KH is low. Tannins and humic acids can push pH down in soft, poorly buffered water, which is exactly what a peat swamp is. In hard, well-buffered tap water the carbonate buffer absorbs it and you get the colour without the chemistry change. Test KH before you assume the leaves are doing anything beyond tinting.

Is wild-caught fish keeping unethical?

Not always, and it is worth being specific rather than blanket. Some ornamental fisheries are poorly regulated and damaging; others, such as the long-established Rio Negro cardinal tetra fishery, provide a livelihood that gives local communities a direct economic reason to keep the forest standing. Captive-bred remains the default for hardiness and welfare reasons, but the ethics vary by species and source.

Can I keep plants in a biotope tank?

It depends which habitat. Blackwater peat swamps are naturally dim and nearly plantless below the surface, so moss and floating cover is closer to the truth than a carpet. West African streams have genuine aquatic plants including Anubias and Bolbitis. Choosing a habitat that suits the planting you want is easier than forcing plants into one that never had them.

How dark should blackwater actually be?

Tea-coloured, not opaque. You should still be able to see across the tank. If the water is so dark you cannot check on the fish, you cannot spot illness early, which is a real welfare problem rather than a stylistic one. Build the colour up gradually and stop when the tank still reads clearly.

Freshwater biotopes that work on tap water