Start With Your Source Water
Before buying equipment or rare livestock, take a closer look at the water you’re filling your tank with.
- Is it hard or soft?
- What are the GH (general hardness) and KH (carbonate hardness)?
- Are you using tap, well, or RO/DI water?
- Does your source water support your aquarium goals, or will it need adjusting?
⚠️ Important: Most “master test kits” don’t include GH or KH. Even many pet stores won’t test for them. Yet these values are critical for plants, shrimp, snails, and overall stability. In saltwater, alkalinity, calcium, and magnesium take their place as the “big three.”
Reality Check: Saltwater vs Freshwater
Saltwater Aquariums
- In reef tanks, RO/DI isn’t optional.
- Unless you live right on the coast and have legal access to natural seawater (NSW), you’ll need purified RO/DI water and a quality salt mix.
- Even where collection is allowed, risks are high (pollution, pathogens, seasonal swings). Some areas ban it outright or require permits.
- Most reef aquarists stick with RO/DI: consistent, safe, and legal.
Freshwater Aquariums
- Results vary depending on your tap water.
- Some aquarists are lucky: their tap matches many common plant needs.
- Others aren’t so lucky: too hard, too soft, or unstable water causes plants to melt.
- In those cases, RO/DI with remineralization is often the only reliable solution.
General Hardness (GH) – Freshwater Only
GH measures dissolved calcium and magnesium.
- Fish: supports bones, scales, and muscle function.
- Plants: promotes strong leaf and root growth.
- Shrimp & snails: necessary for shells and exoskeletons.
Where it comes from: geology (limestone, dolomite, gypsum), tap water, or RO/DI (zero unless remineralized).
Why it varies:
- Mountain runoff = soft water (low GH).
- Limestone aquifers (like much of the U.S. Midwest) = very hard water (high GH).
Carbonate Hardness (KH) – Freshwater Buffer
KH measures carbonates and bicarbonates, which buffer acids and stabilize pH.
Think of KH as your tank’s shock absorber:
- High KH: steady pH, fewer swings.
- Low KH: unstable pH, prone to sudden crashes.
⚠️ Why it matters: KH prevents acids (from fish waste, CO₂, or organics) from dropping your pH too quickly. Without KH, water may test fine in the morning but crash by night.
Saltwater Balance: Alkalinity, Calcium & Magnesium
In reef tanks, the focus shifts from GH/KH to the “big three”:
- Alkalinity (7–11 dKH): buffers pH, fuels coral growth.
- Calcium (400–450 ppm): skeletal structure for corals.
- Magnesium (1250–1350 ppm): stabilizes the balance, prevents precipitation.
⚠️ If these drift, sensitive corals (especially SPS) can lose tissue or stop growing.
pH – The Number Everyone Watches
pH shows if water is acidic or alkaline. Stability matters more than the exact number.
- Fish adapt to a wide pH range if it stays consistent.
- What stresses them is a swing (e.g. 7.8 → 6.4 overnight).
- Buffering capacity (KH in freshwater, alkalinity in saltwater) controls pH stability.
Easy vs Difficult Species
Easy species (adaptable):
- Freshwater: Java fern, Anubias, hornwort, Vallisneria; guppies, mollies, danios.
- Saltwater: Zoanthids, mushrooms, green star polyps.
Difficult species (need stability):
- Freshwater fish: Discus, wild angelfish, Apistogramma.
- Freshwater plants: Ludwigia pantanal, Tonina, Eriocaulons.
- Saltwater corals: SPS corals (Acropora, Montipora), sensitive LPS like Goniopora.
Real-World Ranges (With Plant Matches)
Community Planted Tank (Tetras, Rasboras)
- GH 4–8 | KH 3–6 | pH 6.5–7.4
- Plants: Amazon swords, crypts, vallisneria, Java fern, mosses, Ludwigia, Hygrophila.
Livebearers (Guppies, Mollies, Swordtails, Platies)
- GH 8–12 | KH 6–10 | pH 7.2–8.2
- Plants: Vallisneria, hornwort, Anubias, Java fern, water sprite, mangrove seedlings.
Neocaridina Shrimp (Cherry Shrimp, etc.)
- GH 6–8 | KH 2–6 | pH 6.8–7.6
- Plants: Mosses, Subwassertang, Anubias nana petite, bucephalandra, dwarf sag.
Caridina Shrimp (Crystal, Bee, etc.)
- GH 4–6 | KH 0–2 | pH 5.8–6.8
- Plants: Crypts, Java fern, Bolbitis, mosses, floaters, sensitive stems like Rotala or Tonina.
African Rift Cichlids (Mbuna, Peacock, Tanganyikan)
- GH 10–20 | KH 8–12 | pH 7.8–8.6
- Plants: Vallisneria, Anubias (most setups focus on rockwork).
Reef Tanks (Saltwater)
- Alkalinity 7–11 dKH | Calcium 400–450 ppm | Magnesium 1250–1350 ppm | pH 8.0–8.4
- Macroalgae: Chaetomorpha, Caulerpa, Gracilaria.
Adjusting GH, KH, or Alkalinity
To raise hardness/alkalinity:
- Freshwater: crushed coral, aragonite, GH/KH boosters.
- Saltwater: dosing alkalinity buffers, calcium, magnesium.
To lower hardness/alkalinity:
- Freshwater: dilute with RO/DI water or remineralize.
- Saltwater: handled with balanced dosing and nutrient export.
⚠️ Avoid quick fixes: Chemicals like pH Down don’t remove minerals — they only mask them.
Testing That Matters
Accurate testing is the backbone of stability.
- Freshwater: drop kits for GH/KH.
- Saltwater: digital meters and titration kits for alkalinity, Ca, Mg.
- Strips: fine for quick checks but confirm before adjusting.
⚠️ Reminder: Most “master kits” skip GH and KH. Many pet stores won’t test them either — ignoring these values leads to long-term problems.
Final Thoughts
Stable water chemistry is the single most important factor in long-term aquarium success.
When your parameters are balanced:
- Fish behave naturally and live longer.
- Plants grow lush instead of melting.
- Shrimp molt cleanly and thrive.
- Corals open fully and build strong skeletons.
👉 Next up: We’ll cover water testing in detail and how to use RO/DI properly — when it’s required and how to remineralize for total control.