Water softener salt harming aquarium fish?

Before we talk kit, there's something more urgent here — if your home has a water softener that uses sodium-based salt, the water coming out of your taps is likely high in sodium ions, and that's genuinely problematic for fish. Softened water replaces calcium and magnesium with sodium, which disrupts fish osmoregulation (their ability to regulate fluid balance), and many species simply can't tolerate it long-term. That said, the nitrogen cycle is still the other critical piece: a new tank needs to establish beneficial bacteria before fish go in, and skipping that kills more fish than almost anything else. So — are you starting fresh or adding to an existing setup, what size tank are you thinking, and what fish are you hoping to keep?

Tetras and corydoras are both great choices, but they do need water with proper mineral content — the sodium-heavy output from an ion-exchange softener really isn't suitable for either. The good news is most homes have an unsoftened feed somewhere: your outside tap or the cold supply under the kitchen sink before it hits the softener unit. Do you know if you have an unsoftened tap or a bypass point you could use?

Testing is exactly the right instinct. For tetras and corydoras you want moderately soft to medium-hard water, pH around 6.5–7.5, and low or zero sodium. The API Freshwater Master Test Kit is what I'd recommend here — it covers pH, ammonia, nitrite, and nitrate, which you need anyway to track your nitrogen cycle, and it's accurate enough to give you a real picture of your source water before you add any fish. Test your outside tap water and compare: if the hardness and pH are in range and there's no salt softener touch on that line, you're likely fine to use it.

Yes, always — even unsoftened mains water contains chlorine or chloramine, which will kill your beneficial bacteria and stress fish. Seachem Prime is the one I'd use: it neutralises chlorine and chloramine, detoxifies ammonia and nitrite at low levels (which is a genuine safety net during cycling), and a small bottle treats a huge amount of water. For a 60 litre tank it'll last you a very long time. Just be clear — Prime doesn't fix sodium from a softener, which is why getting the source water right first matters more than any conditioner.

That's exactly right, and it's a solid plan. For cycling a 60 litre tank without fish, add a source of ammonia — pure ammonia drops or a small pinch of fish food daily — and test every couple of days. You're waiting for ammonia and nitrite to both read zero with a nitrate reading rising, which typically takes three to five weeks. Once you're there, do a partial water change and your tank is ready. Tetras and corydoras are forgiving community fish once the water chemistry and cycle are stable — you're setting yourself up well.