Used filter media vs bottled bacteria for cycling

Good news — you're already thinking about this the right way. Before spending a penny on fish, the single most important concept is the nitrogen cycle: your tank needs a colony of beneficial bacteria established before fish go in, and skipping that step is the number one reason new fish die. So: are you starting completely from scratch or adding to something existing, what size tank are you thinking, and what fish are you hoping to keep?

Yes, honestly, used filter media from a healthy established tank is better. It carries live, already-thriving bacterial colonies — not dormant ones sitting in a bottle on a shelf. If your friend's tank is disease-free and well-maintained, a chunk of their sponge or some of their ceramic media dropped straight into your filter will kick-start your cycle faster and more reliably than any bottled product. That's not a knock on bottled bacteria — it genuinely works — it's just that live media is the gold standard, and the fact that it's free makes it the obvious choice here.

One thing you absolutely need to buy is a reliable liquid test kit — and this is the one area I'd tell every budget-conscious beginner not to cut corners. The API Freshwater Master Test Kit is the recommendation here: it tests ammonia, nitrite, nitrate, and pH, gives you around 800 tests in total, and costs about £25-30. That sounds like a lot upfront, but the strips people often buy instead are notoriously inaccurate and cost more per test over time. With your seeded media, your cycle could complete in as little as one to two weeks — but you'll only know it's safe to add fish when you're seeing zero ammonia, zero nitrite, and nitrate starting to climb. Without that kit, you're guessing, and guessing is what kills fish.

Pretty much that, yes. Make sure your heater is holding a stable temperature — around 24-26°C suits both tetras and bettas — and use a dechlorinator like Seachem Prime when you do water changes during the cycle and after. Prime is worth having anyway because it temporarily detoxifies ammonia and nitrite, which gives you a small safety buffer. But the API Master Test Kit is the one non-negotiable purchase. Get the free media from your friend, run the test kit, and you'll be adding fish in a couple of weeks without having wasted money on a bottled bacteria product you didn't need.

That's exactly the right call — you're setting yourself up properly. One last tip: when you get the media, get it into your filter within a couple of hours and keep it wet in some tank water during transport, not tap water. Tap water has chlorine and will kill the bacteria you're trying to keep alive. Good luck with the setup — tetras in a 60 litre will look brilliant once it's cycled.