Hard Water vs. Soft Water Fish: How to Match Species to Your Tap

Two aquarists do everything the same — same tank size, same filter, same diet, same temperature — and one keeps healthy fish for years while the other can't keep cardinal tetras alive past 30 days. The difference is almost always water hardness. It's the most under-discussed parameter in the hobby, and it's the one that quietly decides whether your stocking choices will succeed.

GH and KH: what they mean

"Hardness" actually refers to two distinct measurements:

• GH (general hardness) measures dissolved calcium and magnesium. It's what fish physiologists mean by "hard" water. Measured in degrees (dGH) or ppm.
• KH (carbonate hardness, sometimes called alkalinity) measures dissolved bicarbonate and carbonate. It buffers pH against swings. Also in degrees (dKH).

Both matter, but for stocking decisions, GH dominates.

Typical ranges

• Very soft: 0-3 dGH (0-50 ppm). Rainwater, RO water, blackwater rivers.
• Soft: 4-8 dGH (70-140 ppm). Amazon basin, much of Southeast Asia.
• Moderately hard: 9-12 dGH (160-210 ppm). Most US tap water.
• Hard: 13-20 dGH (230-350 ppm). Limestone-fed regions, much of the desert Southwest.
• Very hard: 20+ dGH (350+ ppm). Lake Malawi/Tanganyika source water.

You can find your tap GH on your municipal water quality report (often listed as "total hardness"), or test it with a GH/KH liquid test kit.

Why hardness matters more than pH

Fish can usually adapt to a pH outside their ideal range, but they cannot adapt to wrong-hardness water long-term. Soft-water species in hard water have trouble regulating osmotic pressure across their gills, develop kidney damage over months, and rarely breed. Hard-water species in soft water leach minerals from their bones and develop "hole-in-the-head" and lateral line erosion. The damage is slow and invisible — fish look fine for months before declining.

This is why some aquarists who chase pH with chemicals end up making things worse: they alter pH temporarily but don't change the underlying hardness, so the fish stay stressed.

Species that thrive in soft water (0-8 dGH)

Originally from the Amazon, the blackwater rivers of South America, and Southeast Asian peat swamps. These species need soft water for long-term health and absolutely require it to breed.

• Cardinal tetras, neon tetras, rummynose tetras, ember tetras, most other tetras
• Discus, angelfish (wild-type), most South American dwarf cichlids (apistogramma, rams)
• Bettas, licorice gouramis, chocolate gouramis
• Otocinclus, most plecos (especially L-numbers)
• Crystal red shrimp, taiwan bee shrimp, and other Caridina shrimp

Species that thrive in hard water (13+ dGH)

From limestone-influenced lakes and rivers. These species need hard, alkaline water and will struggle in soft water.

• All African Rift Lake cichlids (Lake Malawi mbuna, peacocks; Lake Tanganyika tropheus, shell-dwellers, frontosa)
• Livebearers — guppies, mollies, platies, swordtails
• Goldfish (tolerate a wider range but prefer harder water)
• Mystery snails, nerite snails (need calcium for their shells)

Species that adapt to moderate water (8-15 dGH)

The most forgiving — and the smart default if you don't want to alter your tap water.

• Most danios (zebra, pearl, leopard)
• Corydoras catfish
• Honey gouramis, dwarf gouramis
• Most barbs (cherry, tiger, gold)
• Cherry shrimp and other Neocaridina shrimp
• Rainbowfish (most species)
• Kuhli loaches

Changing your water's hardness — when and how

If you really want to keep a species that doesn't suit your tap water, you have two options:

• Soften hard water: Use reverse osmosis (RO) or deionized (DI) water, optionally mixed with tap water to dial in the target hardness. Add minerals back to RO water using a product like Seachem Equilibrium or Salty Shrimp GH+. Best for soft-water shrimp and dedicated Amazon biotopes.
• Harden soft water: Add crushed coral or aragonite to the substrate or filter, or dose Seachem Equilibrium. Best for African cichlid tanks where your tap is too soft.

For most aquarists, the better strategy is the reverse: match species to your water. Your tap water is consistent, free, and doesn't require ongoing chemistry. Look up your municipal GH, then stock species that thrive in that range.

Quick test before you stock

Test your tap water with an API GH & KH test kit. Run the test on a glass of water sitting on your counter, not just from the faucet (water from cold pipes can read slightly differently than aged water). The result will tell you, more accurately than any other single test, which species will succeed in your tank.

Foundations: the nitrogen cycle →   Why fish die after water changes →

Frequently asked

What is the difference between GH and KH in aquariums?

GH (general hardness) measures dissolved calcium and magnesium and determines which species suit the water. KH (carbonate hardness) measures bicarbonate/carbonate and buffers pH against swings. Both are reported in dGH/dKH or ppm.

Can I keep neon tetras in hard water?

Neon tetras tolerate moderate hardness short-term but their health suffers in hard water (above 12 dGH) long-term. They'll rarely breed and have shortened lifespans. Soft to moderately soft water (2-10 dGH) is ideal.

What fish are best for hard tap water?

Livebearers (guppies, mollies, platies, swordtails), African Rift Lake cichlids, and goldfish all thrive in hard water (13+ dGH) and prefer alkaline pH.