This is a specific conversation about Fish aquarium setup — getting started right — ask about yours and get one honest answer for your exact situation.
inparticular.ai
That's a frustrating one — and unfortunately more common than it should be. Copper is lethal to beneficial bacteria, so even a small amount of copper-treated gravel mixed into your tank can completely wipe out a developing colony, which is exactly what happened to you. The good news is your tank isn't lost; you just need to restart the cycle with something that isn't working against you. Before we get into fixing it: what size tank are you running, and what fish were you planning to keep once it's cycled?
It's a 120 litre tank. I was going to put in some tropical community fish — tetras, a couple of corydoras, maybe some guppies. I still have the gravel from my friend's tank but I threw out the fish store stuff after I figured out what went wrong.
inparticular.ai
Good call removing the fish store gravel — even a small contaminated portion can stall a cycle indefinitely. The gravel from your friend's tank should still have viable bacteria on it, especially if you kept it damp and got it back in the tank quickly. What I'd want to know is: how long was it out of water, and did you rinse it at any point? That'll tell us how much living biology you've actually got left to work with.
It was out of the water for maybe two hours in a bag, and I didn't rinse it — I'd read not to. But that was about three weeks ago and my ammonia is still not dropping at all when I do a fishless cycle with ammonia drops.
inparticular.ai
Two hours unrinsed is borderline — some bacteria survive, but the colony is likely sparse and stressed. Three weeks with no ammonia drop tells you it's not enough to carry the cycle on its own. The fix here is a good bottled bacteria product to boost what you've got. I'd go with Seachem Stability — it's genuinely one of the more reliable options, it works across a wide temperature and pH range which matters for a tropical community setup, and it's specifically designed to establish both the ammonia-oxidising and nitrite-oxidising bacteria you need. Dose it daily for seven days as directed, keep dosing your ammonia to around 2–3 ppm each day, and test every 24–48 hours. You should start seeing ammonia drop within a week if conditions are stable.
Is Seachem Stability safe to use while the gravel from my friend's tank is still in there? I don't want to interfere with whatever bacteria might still be alive on it.
inparticular.ai
Completely safe — Seachem Stability is essentially adding to what you already have, not replacing it. It won't compete with or harm any surviving bacteria on your friend's gravel; it just populates the surfaces where bacteria are sparse or missing. Think of it as reinforcing the existing colony rather than starting over. One thing to check: make sure your ammonia source is pure ammonia with no surfactants or fragrances — Dr Tim's Ammonium Chloride is the cleanest option if you're not already using it, but any clear, unscented ammonia works. Keep your filter running continuously and don't do any water changes until ammonia and nitrite are both reading zero on back-to-back days.
That's really helpful. One more thing — once the cycle is done, is there anything I should do differently when I add fish given what happened?
inparticular.ai
Yes — add fish in small groups rather than all at once, even with a fully cycled tank. Your bacterial colony is sized to the ammonia load it's been processing, so a sudden large jump in bioload can temporarily crash nitrite levels. Start with your corydoras or a small group of tetras, wait two weeks, test your water, then add the next group. Keep Seachem Stability on hand for the first few weeks and dose it any time you add new fish — it helps the bacteria population catch up quickly. With a 120 litre tank and that mix of fish you'll end up with a really solid little community setup.
