Killifish biotope tank cycling advice

A killifish biotope is a brilliant project — but before you buy a single piece of driftwood or bag of aqua soil, the nitrogen cycle is the thing you absolutely must get right first. Skipping or rushing it is the number-one reason fish die in new tanks, and with killifish it's even more critical because the soft, acidic water you'll need slows bacterial colonisation and changes how ammonia behaves. So — are you starting from scratch or working with an existing tank? What size are you thinking, and which killifish species are you planning to keep?

Your concern is completely valid. Nitrifying bacteria — particularly Nitrobacter and Nitrospira — do colonise more slowly below pH 6.5, and ammonia becomes less acutely toxic but also harder to test accurately in soft water. The practical answer is to cycle at a slightly higher pH, around 6.8 to 7.0, let the cycle complete fully, then gradually acidify with botanicals, catappa leaves, or a peat-filtered water change once ammonia and nitrite are reading zero. For Aphyosemion australe in 60 litres that's very manageable. What are you using to achieve the soft, acidic water — RO, rainwater, or something like a blackwater extract?

Exactly right — pure RO has almost no buffering capacity and the pH will swing wildly, which stresses bacteria even more than low pH does. Remineralise to a low GH of around 3 to 5 degrees and keep pH at 6.8 during the cycle using a small amount of your blackwater conditioner. The product I'd recommend for kickstarting the cycle in these conditions is API Quick Start — it contains live Nitrosomonas and Nitrospira strains that establish faster than waiting for ambient colonisation, which matters a lot when your water chemistry is on the softer end. Dose it at the start, dose again after any large water change, and run the cycle with a small ammonia source like pure ammonia drops rather than fish food, so you have clean, predictable readings. You should see a complete cycle in two to three weeks.

Wait until after — or at most add a small quantity toward the end of the cycle once ammonia and nitrite are trending toward zero. Botanicals release tannins and organic acids that lower pH and add to the organic load, both of which can interfere with bacterial establishment and muddy your test results. Once the cycle is confirmed complete — ammonia 0, nitrite 0, nitrate rising — then start adding catappa leaves and botanicals gradually while monitoring pH. That way you're acidifying a biologically stable tank rather than trying to cycle an unstable one. Your Aphyosemion australe will thank you for the patience.

Dr Tim's Ammonium Chloride is the one to use — it's a pure, phosphate-free solution designed specifically for fishless cycling, which means no additives that could affect your soft-water chemistry. Dose to 2 ppm ammonia to start the cycle. In soft, low-buffered water keep your test kit handy because pH can drift during cycling and you may need a tiny carbonate buffer addition to keep it stable above 6.5 until the bacteria are established. Once you're cycled, acidify slowly with your botanicals and you'll have a beautiful, stable biotope ready for the killies.