pH 8.4 tap water & acidic fish cycling problems

pH 8.4 tap water is a genuinely tricky starting point — beneficial bacteria for acidic fish species like tetras, discus, or apistogrammas establish much more slowly at that pH, and the water chemistry is actively working against you before you've even added fish. The nitrogen cycle is the single most important concept to nail before anything else; skipping or botching it is why most new tanks crash. So — are you starting from scratch or working with an existing tank, roughly what size are you looking at, and which fish species are you trying to keep?

That stalling ammonia makes complete sense given your pH. Nitrifying bacteria — especially Nitrosomonas — slow down significantly above pH 8.0, and the bacteria that thrive in acidic blackwater environments really do struggle to colonise at 8.4. The issue isn't your cycling method, it's that you need to bring the pH down before you cycle, not after. What are you currently using to try to lower the pH, if anything?

Peat alone rarely has enough buffering capacity to fight alkaline tap water — it softens but doesn't hold the pH because your tap water's carbonate hardness (KH) keeps pushing it back up. You need to address the KH first, then adjust pH. The product I'd point you to is Seachem Acid Buffer. It works by reducing carbonate alkalinity and pushing pH down toward the acidic range in a controlled way, and crucially it maintains a stable target — usually around 6.8 to 7.0 — rather than letting it drift back. For cardinal tetras you're aiming for pH 6.5 to 7.0, so Seachem Acid Buffer paired with their Neutral Regulator (to set the ceiling) gives you real control. Start your cycle only once you've held a stable pH for 48 hours. A 300g tub will easily cover a 120 litre tank through the whole cycling process and beyond.

Do a 50% water change first — it halves the carbonate load you're fighting against, which means the Acid Buffer works faster and you use less of it. Then dose according to the instructions for your target pH, test after 24 hours, and adjust. Don't rush it; a pH shift of more than 0.2 per day stresses bacteria and fish alike, so bring it down gradually over two or three days. Once you're sitting stable at 6.8 to 7.0 for two full days, add your ammonia source and start the cycle properly. With stable pH, you should see nitrite appear within 7 to 10 days.

Yes, worth adding once pH is stable — not before, because the same pH problem that slowed your natural cycle will also reduce the effectiveness of bottled bacteria. Seachem Stability is a good pairing with the Acid Buffer; it's shelf-stable, works across a reasonable pH range, and seeds your filter media reliably. Dose daily for the first seven days of your cycle. Between the stable pH, Stability, and a proper ammonia source (pure ammonia like Dr Tim's, not cleaning products), you should get a full cycle in three to four weeks rather than watching it stall indefinitely.