Ammonia cycle stalled? Surfactants explained

You've almost certainly diagnosed it correctly — surfactants in hardware store ammonia are the number one reason fishless cycles stall or never get going. Even a tiny amount of surfactant is toxic to the nitrifying bacteria you're trying to cultivate, and the frustrating part is there's often no way to tell from the label whether it's in there. The only reliable fix is to start with ammonia that's specifically formulated for fishless cycling. Before we talk about that — how far into the cycle were you, and what size tank are we working with?

Good call doing the shake test — that's exactly how you confirm it. On whether to start from scratch: if you've done a large water change and removed most of the contaminated ammonia, your filter media may still hold some early bacterial colonies, but honestly after three stalled weeks there's unlikely to be much worth saving. I'd do another large water change — get it up to around 80% — then dose with a clean ammonia source and treat it as a fresh start. The good news is you're not losing weeks of progress; a proper cycle with the right ammonia typically runs 4–6 weeks anyway. What fish are you planning to keep once it's done?

Perfect combination for a 200 litre — you've got room to do that nicely. For the cycle itself, the product I'd point you straight to is Dr Tim's Ammonium Chloride. It's a pure ammonium chloride solution with absolutely nothing else in it — no surfactants, no fragrances, no additives — and it's specifically designed for fishless cycling. It comes with a dosing guide so you know exactly how much to add per litre to hit the ammonia concentration you're targeting, which takes the guesswork out completely. A lot of people also pair it with Dr Tim's One & Only live bacteria to seed the filter and shave a week or two off the cycle time, but the ammonium chloride alone will get you there reliably. It's available on Amazon UK and a single bottle will comfortably handle your 200 litre tank.

I'd keep the filter media in if it's been running in the tank throughout — even a weak bacterial colony is better than nothing, and surfactants inhibit growth rather than necessarily killing everything outright. Rinse the media lightly in tank water (never tap water), leave it in your filter, and let the fresh Dr Tim's ammonia give those bacteria something clean to work with. If you see ammonia starting to drop within the first week, you've got survivors worth building on. If it sits flat again past day ten, that's when I'd consider replacing the media entirely and leaning on a bottle of live bacteria to seed it properly.

You'll get there this time — you clearly know what you're doing, you just had bad source material. Keep a cheap liquid test kit on hand to track ammonia, nitrite, and nitrate as you go, and once you see ammonia and nitrite both crashing to zero within 24 hours of dosing, you're cycled and ready for fish. Good luck with the community tank — that's a really enjoyable mix to keep.