pH crash during tank cycle — soft water fix

A pH crash mid-cycle with very soft, acidic tap water is one of the more frustrating setups problems you can hit — the beneficial bacteria that process ammonia need a stable pH above 7 to thrive, and when your water has almost no buffering capacity, pH can nosedive fast and effectively stall the whole cycle. Before buying anything else, tell me: what size tank are you cycling, how far in are you, and what fish are you planning to keep? That last part matters a lot because some species actually prefer soft acidic water, which changes the approach completely.

Right, so the good news is that tetras and corydoras genuinely like soft acidic water, so you don't want to overdo any buffering — you just need enough stability to get the cycle finished, not to push the tank alkaline. At pH 6.2 the ammonia-oxidising bacteria are struggling because their enzyme activity slows significantly below 6.5. The culprit here is almost certainly zero carbonate hardness, meaning there's nothing in your water to resist the acidification that naturally occurs as nitrification produces nitric acid. You need a small, controlled KH boost — just enough to hold pH in the 6.8–7.2 range while the cycle completes, after which you can let it drift back toward what your fish prefer.

The product I'd point you to is Seachem Equilibrium — but actually for your specific problem, what you want alongside it is Seachem Alkalinity Buffer (sodium bicarbonate-based), or more precisely, Seachem's Acid Buffer and Alkaline Buffer used in combination. That said, the single clearest fix for a cycling tank with your water profile is crushed coral or aragonite substrate added in a media bag in your filter. I'd recommend Carib Sea AragaMight or a generic crushed coral in a mesh bag — it dissolves slowly, raises KH gently, and self-regulates because it stops dissolving once pH is stable around 7.2. It won't overshoot, which is exactly what you want. Add about 200–300g in your filter basket, and within 48 hours your pH should stabilise and the cycle should start moving again.

That's the elegant part — crushed coral is self-limiting. It only dissolves when pH drops below around 7.2 to 7.4, so it acts like a buffer floor rather than a ceiling-pusher. Once your cycle is complete and the tank is established, you can reduce the amount gradually over a few weeks and let pH drift back toward 6.5–7.0, which your tetras and cories will love. You're not permanently altering the tank chemistry, you're just giving the bacteria enough foothold to finish their job. The coral also adds a small amount of calcium and magnesium which does no harm at all at low levels.

Search for Carib Sea AragaMight on Amazon UK — it comes in a 1.8kg bag which is more than enough for your 120 litre tank. You'll use roughly 250g in a fine mesh media bag (a spare filter sock works well) placed directly in your filter. The bag makes it easy to reduce or remove the amount once the cycle is done. It typically costs around £10–14 for that bag size, so it's a cheap fix for a problem that can otherwise stall a cycle for weeks or force you to start over. Keep testing pH every day for the first few days after adding it — you should see it stabilise within 24 to 48 hours and ammonia conversion should pick back up within the week.