Your fish live in their water. Not on it, not beside it. Every gill movement pulls dissolved minerals into their bloodstream. Every moult, every egg, every colour change depends on what's dissolved in the tank. The water isn't a container. It's an ingredient.
Most fishkeepers test for ammonia and nitrite (rightly so), then stop. But the mineral composition of your water determines whether shrimp can moult safely, whether cichlids develop strong bones, whether plants photosynthesise properly, and whether your pH stays stable between water changes. Understanding what's in your water, and what should be, is the difference between a tank that survives and one that thrives.
GH, KH, TDS, and pH: what each one actually measures
These four abbreviations appear on every care sheet, often without explanation. They're related but measure different things.
General hardness (GH) measures the combined concentration of calcium and magnesium ions. It's reported in degrees of hardness (°dH) or parts per million (ppm) as calcium carbonate equivalent. One degree GH equals roughly 17.8 ppm. GH tells you how much of the two minerals that matter most for bones, shells, and exoskeletons are available in the water. A GH of 6 °dH is moderately soft; 12 °dH is moderately hard.
Carbonate hardness (KH) measures bicarbonate and carbonate ions, the water's buffering capacity. This is the number that determines how stable your pH is. Water with a KH of 1 °dH has almost no buffer: add a small amount of acid (from CO2, fish waste, or tannins) and the pH plummets. Water with a KH of 10 °dH resists pH change stubbornly. KH doesn't measure the same minerals as GH, despite sharing the same unit. They're independent values.
Total dissolved solids (TDS) is a catch-all measurement of everything dissolved in the water: minerals, salts, organic compounds, the lot. It's measured in ppm with a conductivity meter. TDS is useful as a quick health check (has something changed?) but it doesn't tell you what's dissolved, only how much. A TDS of 200 ppm could be mostly calcium, or mostly sodium, or mostly fish waste. Context matters.
pH measures how acidic or alkaline the water is, on a scale from 0 to 14. Pure water sits at 7.0 (neutral). Most tropical fish want somewhere between 6.0 and 8.0, depending on species. The crucial thing about pH is that it's a result, not an input. You don't set pH directly. It follows from the balance of acids and buffers in your water, which is why KH matters so much.
The relationship between these four values trips people up. GH and KH are independent: you can have high GH with low KH, or the reverse. TDS includes both but also includes things that aren't minerals. pH is influenced by KH (the buffer) and by acids in the system (CO2, tannins, biological waste). Once you stop treating them as a single "water quality" number and start reading them as separate measurements, water chemistry becomes much more manageable.
The six ions and what they do in an aquarium
The Water Dictionary tracks six mineral ions. Here's what each one does for aquatic life.
Calcium (Ca) is the structural mineral. Fish use it for bones, scales, and gill function. Shrimp and snails need it to build exoskeletons; without enough calcium, shrimp moults fail (the infamous "white ring of death" in dwarf shrimp, where the old shell cracks but the new one hasn't hardened). Axolotls need it for cartilage development and gill integrity. Plants use calcium for cell wall structure.
Magnesium (Mg) is the enzyme mineral. It's a cofactor in hundreds of biological processes: photosynthesis in plants, nerve function in fish, and crucially, calcium absorption. Without adequate magnesium, organisms can't use the calcium that's available. In planted tanks, magnesium deficiency shows as yellowing leaves with dark veins (chlorosis). The typical calcium-to-magnesium ratio in freshwater is around 3:1 by mass.
Bicarbonate (HCO₃) is the buffer. It resists pH change by neutralising acids. In aquariums with CO2 injection, bicarbonate determines how far the pH drops when CO2 dissolves (forming carbonic acid). For African cichlids, high bicarbonate keeps pH stable in the alkaline range their biology requires. For blackwater species, low bicarbonate allows the acidic conditions they've evolved in. Bicarbonate is the ion behind KH.
Sodium (Na) plays a minor role in freshwater aquariums. Small amounts support osmoregulation (the process by which fish maintain their internal salt balance against the surrounding water). In practice, sodium is rarely discussed in fishkeeping because most freshwater species need very little, and most water sources provide enough without intervention.
Chloride (Cl) helps protect fish against nitrite toxicity by competing for the same uptake pathway in the gills. A baseline of around 10–20 mg/L is beneficial. Beyond that, chloride isn't something freshwater keepers typically need to think about. (Don't confuse chloride, a harmless ion, with chlorine or chloramine, which are toxic disinfectants found in tap water.)
Sulphate (SO₄) is the least discussed ion in aquarium keeping. It's present in most water sources and contributes to TDS, but there's no established target for freshwater aquariums. Where our brewing profiles use sulphate as a major flavour lever, aquarium profiles treat it as background noise. We've set low default values and noted the data gap.
Common species groups and their requirements
Different species evolved in different water. A shrimp from a Southeast Asian stream and a cichlid from an African rift lake need fundamentally different mineral profiles. The table below summarises the target profiles available on this site.
| Profile | GH (°dH) | KH (°dH) | pH | TDS (ppm) | Character |
|---|---|---|---|---|---|
| Neocaridina Shrimp | 8 | 4 | 7.0–7.6 | 150–300 | Moderate hardness; stable buffering for safe moults |
| Caridina Shrimp | 5 | 1 | 6.0–6.8 | 100–180 | Soft, acidic, minimal buffering |
| Betta Splendens | 5 | 3 | 6.5–7.5 | 80–200 | Soft to moderate; adaptable but prefers gentle water |
| General Community Tropical | 6 | 4 | 6.5–7.5 | 150–300 | The compromise zone for mixed tanks |
| Planted Tank (CO2) | 6 | 4 | 6.0–7.0 | 150–250 | Enough minerals for plant growth; KH controls CO2/pH |
| African Cichlids | 12 | 10 | 7.8–8.6 | 300–450 | Hard, alkaline, strongly buffered |
| Discus | 4 | 3 | 6.0–7.0 | 100–250 | Soft; breeding requires even softer |
| Blackwater Biotope | 2 | 1 | 5.0–6.5 | 30–80 | Very soft, acidic, mineral-poor |
| Axolotl | 10 | 5 | 7.0–7.8 | 150–300 | Moderately hard; reflects Lake Xochimilco |
Each profile links to a dedicated page with the full mineral breakdown and blending ratios by region.
Why tap water is unreliable for sensitive species
Tap water is treated for human safety, not aquarium suitability. That's not a criticism; it's just a different set of priorities.
The first problem is chloramine. Most municipal suppliers now use chloramine (a chlorine-ammonia compound) instead of free chlorine because it's more persistent in the pipe network. Chloramine is toxic to fish and shrimp at any concentration. It must be neutralised with a dechlorinator before the water goes anywhere near a tank. Free chlorine off-gasses if you leave water standing; chloramine doesn't.
The second problem is variability. Tap water mineral content changes with the seasons, with maintenance on the supply network, and with the blend of sources your utility draws from. A keeper in London might see their GH swing by 2–3 °dH across the year. For hardy community fish, that's a non-issue. For Caridina shrimp, which can die from a GH shift of 2 °dH during a water change, it's a serious risk.
The third problem is heavy metals. Copper, zinc, and lead can leach from household plumbing, particularly in older buildings or after water has been standing in pipes overnight. Copper is lethal to invertebrates at concentrations well below what's safe for humans.
None of this means tap water is unusable. For many community setups, dechlorinated tap water works fine. But for species that need specific, repeatable mineral profiles, tap water is a moving target.
Bottled water blending as an alternative to RO
The standard advice for precise water chemistry is to buy a reverse osmosis (RO) unit, strip everything out, then add minerals back with a remineralising powder. This works. It's also expensive, space-consuming, produces waste water, and requires ongoing maintenance. For a 200-litre tank with weekly water changes, you're processing a lot of water through a slow system.
Bottled water offers a shortcut. The mineral composition of bottled water is fixed by the source and printed on the label. It doesn't change seasonally. It doesn't contain chloramine or copper. And by blending two or three bottles in the right ratio, you can hit a target mineral profile without any specialist equipment.
This is the same approach The Water Dictionary uses for brewing, coffee, and baking: take waters with known mineral contents and combine them to reach a target. Our solver calculates the best blend for each target profile using bottled waters available in your region.
There are honest limitations. Cost is the main one. For a small shrimp tank (30–50 litres) with modest water changes, bottled water is entirely practical. For a 500-litre cichlid tank, the economics shift. You should do the maths for your own setup. Availability is another factor: our blends use widely stocked supermarket brands, but stock varies by location.
For keepers who already own RO systems, our profiles still serve as a reference for what to remineralise to. The mineral targets are the same regardless of how you reach them.
Getting started
Pick the target profile that matches your species. If you're keeping cherry shrimp, start with the Neocaridina profile. If you're running a mixed community tank, the General Community Tropical profile is your starting point.
Check the blending ratios for your region. The Water Dictionary generates water blends using bottled waters available in the UK, US, France, and Italy. The blend will look something like "3 parts Brand A : 1 part Brand B". Mix before adding to the tank, not in it.
Test your blend with a TDS meter to confirm it's in the expected range. TDS meters cost under a tenner and take two seconds to use. They won't tell you the full mineral breakdown, but they'll confirm you haven't made an obvious error.
For your first water change with blended water, go slowly. Replace 10–15% of the tank volume and observe your livestock for 24 hours. If everything looks normal, continue with your regular water change schedule using the blend. The goal is consistency: the same water, every time, with no seasonal surprises.
Water is the environment your fish live in. Start treating it like one.