If you've ever tested your aquarium water and stared at a "GH" and "KH" result with no idea what either number means, you're not alone. They're two of the most skipped-over readings on a standard test kit — mostly because nobody explains them in plain English.
What GH actually is
GH stands for general hardness, and it measures the amount of dissolved calcium and magnesium in your water. Higher GH means "harder" water with more minerals; lower GH means "softer" water with fewer.
This matters because different fish, plants and invertebrates evolved in wildly different water conditions. Livebearers like guppies and platies generally do well in harder water, while many soft-water species — including a lot of tetras, and especially shrimp — are far more sensitive and can struggle or fail to breed in water that's too hard or too soft for their needs.
What KH actually is (and why it's arguably more important)
KH stands for carbonate hardness, and it measures your water's buffering capacity — essentially, its ability to resist sudden pH swings. Think of it like a shock absorber: the higher your KH, the more your water can "absorb" acid-producing waste before your pH crashes.
Low KH is the reason some tanks experience sudden, unexplained pH crashes weeks or months after setup — the water simply ran out of buffering capacity. This is one of the more common invisible causes of a "healthy" tank suddenly going wrong, and it's rarely checked until after something's already stressed.
Why this matters more for shrimp and plants than most people realise
Shrimp are particularly sensitive to GH, since they use dissolved minerals for shell development during moulting — too little GH and shrimp can struggle to moult successfully. Plants are more forgiving generally, but heavily planted tanks can gradually pull GH down over time as they consume minerals, which is worth knowing if you're wondering why your readings drift over the weeks.
Finding out what's actually coming out of your tap
Because water hardness varies by location, season and treatment, the most reliable way to know your starting point is to test your own tap water directly with a liquid test kit (like the API GH/KH test), rather than relying on a general figure for "Sydney" as a whole. Sydney Water also publishes water quality reports for different supply zones on their website, which can be a useful cross-check alongside your own test results.
Our take
Once you know your baseline GH and KH, choosing fish, shrimp or plants becomes far less guesswork — you're matching livestock to the water you actually have, rather than hoping for the best. If you're not sure how to read your results or what to do about them, send us your numbers and we'll help you figure out your next step.