Axolotl

Axolotls are having a moment, and the care advice floating around social media ranges from excellent to dangerously wrong. Water chemistry is one area where getting it right matters more than most keepers realise. Axolotls aren't fish; they're fully aquatic salamanders with exposed external gills and permeable skin. What's dissolved in the water passes directly into their bodies.

Their native habitat is Lake Xochimilco in the Valley of Mexico: high-altitude, mineral-rich, alkaline water fed by mountain springs. This isn't soft, acidic water. Axolotls need a GH of 7–14 °dH (harder than most tropical fish), a KH of 3–8 °dH, and a pH of 7.0–7.8. Soft water damages their gill filaments and compromises the mucus coat that protects them from infection. Acidic water (below pH 6.5) causes direct tissue damage.

The other thing that sets axolotls apart is temperature. They're cold-water animals, comfortable at 16–20°C and stressed above 23°C. This means they can't share a tank with most tropical fish, and it means water changes need temperature matching. Blended bottled water at room temperature is typically fine, but check with a thermometer before adding it to the tank. A 4°C temperature shock is as dangerous as a mineral shock.