Aquarium Water Chemistry at a Glance
Understanding water chemistry is the single most important skill in aquarium keeping. These four numbers determine whether your fish thrive or struggle.
Water Parameter Toxicity Risk
Water Parameters by Fish Type
| Parameter | Community Fish | Discus/Cichlids | Marine Reef |
|---|---|---|---|
| pH | 7.0-7.8 | 6.0-7.5 / 7.8-8.5 | 8.1-8.4 |
| Ammonia | 0 ppm | 0 ppm | 0 ppm |
| Nitrite | 0 ppm | 0 ppm | 0 ppm |
| Nitrate | <40 ppm | <20 ppm | <10 ppm |
| Temperature | 74-80°F | 76-86°F | 76-82°F |
Tank Maintenance Routine
Do's and Don'ts
| Buy a liquid test kit (API Master Kit) — NOT test strips | |
| Test ammonia and nitrite every 3 days during cycling | |
| Test weekly after the tank is established | |
| Make water changes the same temperature as the tank | |
| Use dechlorinator with EVERY water change | |
| Add fish to a tank with any detectable ammonia or nitrite | |
| Do large (50%+) water changes — they destabilize chemistry | |
| Use test strips — they are inaccurate and unreliable | |
| Chase a specific pH number — stability matters more than the number |
💡 Water chemistry is the language your fish use to tell you if they're healthy or struggling. The 'New Tank Syndrome' that kills most beginner fish is simply ammonia poisoning — completely preventable with testing.
⚠️ Most fish deaths in the first month are caused by ammonia toxicity from adding fish to an uncycled tank. Never trust 'instant cycle' products — they can help but don't replace the 4-8 week biological cycle.
Fish don't live in water -- they live in a precise chemical environment that water contains. pH (acidity/alkalinity), hardness (dissolved minerals), ammonia, nitrite, and nitrate are the parameters that determine whether that environment supports life or slowly degrades it. Understanding what these numbers mean, how they interact, and how to test and adjust them is the foundation of fishkeeping. Most beginner fish deaths trace to one or more parameters being outside the acceptable range.
pH & Hardness Preferences by Species Group
| Species Group | pH Range | GH (dGH) | Water Type |
|---|---|---|---|
| Amazon tetras, Discus | 5.5–6.5 | 1–4 | Soft, acidic |
| Most tropical community fish | 6.5–7.5 | 4–12 | Neutral |
| Goldfish, livebearers | 7.0–7.5 | 8–15 | Moderately hard |
| African cichlids | 7.8–9.0 | 12–25 | Hard, alkaline |
Ammonia spike
Nitrite rises
Nitrate appears
NH₃=0, NO₂=0
A beginner fishkeeper typically buys a fish, puts it in a tank of tap water, and wonders why it died. The water looked clear. The filter was running. The problem wasn't visible -- it was chemical. Tap water chemistry varies widely by location, chlorine and chloramine make it toxic to fish until dechlorinated, and the nitrogen cycle that converts fish waste into harmless compounds hadn't been established. These aren't obscure details -- they're the fundamentals.
pH: The Scale, the Stability, and the Species Match
pH is a logarithmic scale from 0-14 measuring hydrogen ion concentration, with 7 being neutral, below 7 acidic, and above 7 alkaline. Because it's logarithmic, pH 6 is 10x more acidic than pH 7; pH 5 is 100x more acidic. Most freshwater tropical fish tolerate pH 6.5-7.5. Discus and most Amazonian species prefer 5.5-6.5. African cichlids require 7.8-9.0. Goldfish do best at 7.0-7.5.
Stability matters more than the exact value within the acceptable range. A fish that has adapted to pH 7.8 and is suddenly placed in pH 6.5 may go into shock from the 16x difference in hydrogen ion concentration -- even though both values are within "acceptable" ranges for many species. Match new fish to the tank's established pH or acclimate them slowly over hours.
Hardness: KH and GH Are Different Things
General hardness (GH) measures dissolved calcium and magnesium ions. Soft water (under 4 dGH) suits tetras, discus, and most Amazonian species. Hard water (over 12 dGH) suits African cichlids, livebearers, and goldfish. Carbonate hardness (KH, also called buffering capacity or alkalinity) measures the tank's resistance to pH swings. Low KH water has less buffering capacity -- the pH can crash rapidly from fish waste acids. This is why the pH in a low-KH planted tank might be stable at setup but crashes over weeks as the fish waste acidifies the water and the low KH can't buffer it.
Soft water aquarists maintaining low KH must monitor pH closely. Adding crushed coral, aragonite, or sodium bicarbonate raises both KH and pH. Peat moss, catappa leaves (Indian almond leaves), or RO water lower KH and pH for discus and other soft-water species.
The Nitrogen Cycle: The Single Most Important Concept
Fish excrete ammonia (NH3) as their primary waste product -- gill-diffused, not urinated. At pH above 7, a significant proportion of ammonia is in the toxic un-ionized form (NH3). At pH below 7, more is in the less-toxic ionized form (NH4+). This is why pH matters beyond species preference: the same ammonia concentration is more dangerous in alkaline water than in acidic water.
Beneficial bacteria (Nitrosomonas converts NH3 to NO2-; Nitrospira converts NO2- to NO3-) colonize the filter and substrate over 4-8 weeks. Until the cycle completes, any ammonia produced can kill fish. Test kits for ammonia, nitrite, and nitrate are essential for cycling and ongoing monitoring. In a cycled tank: ammonia = 0, nitrite = 0, nitrate = below 20 ppm (freshwater) or below 5 ppm (marine reef). Nitrate is removed via water changes (25-30% weekly) and live plants.
Chlorine, Chloramine, and Dechlorination
Municipal tap water contains chlorine or chloramine (chlorine + ammonia) added to prevent bacterial growth in pipes. Both are toxic to fish and to the beneficial bacteria of the nitrogen cycle. Chlorine dissipates from still water in 24 hours; chloramine does not. Dechlorinators (sodium thiosulfate for chlorine, sodium thiosulfate + sodium hydroxymethane sulfinate for chloramine) neutralize both instantly. Seachem Prime is a widely used dechlorinator that also temporarily detoxifies ammonia and nitrite in emergency situations -- adding a double dose can buy 24-48 hours during an ammonia spike before crisis-level toxicity occurs.
Water chemistry parameters for freshwater fish species are established in published ichthyology and aquarium biology literature. The logarithmic nature of pH and its implications for toxicity are established in basic aquatic chemistry. Nitrosomonas and Nitrospira as the key nitrifying bacteria in aquarium biofilters is established in published microbiology (Hovanec and DeLong, 1996, Applied and Environmental Microbiology).
Seachem Prime active ingredients and mechanism (sodium thiosulfate + sodium hydroxymethane sulfinate) are published in the product's SDS and independently verified. The ASPCA aquatic animal guidance and standard fishkeeping references note pH stability as a higher welfare priority than achieving a specific value within the acceptable range for a given species.
- Sources: Merck Veterinary Manual (merckvetmanual.com); ASPCA (aspca.org)