Fish tank cycle failed? Here's why and how to fix it

Eight weeks of waiting and nothing happened — that's genuinely frustrating, but there's a clear reason: the nitrogen cycle needs an ammonia source to get started. Beneficial bacteria have nothing to feed on without ammonia, so they simply never colonise the tank. You haven't done anything wrong with the process itself, you were just missing the one ingredient that drives the whole thing. Before we talk about restarting, can you tell me what size tank you're working with and what fish you're hoping to keep? That'll shape the best approach for you.

That's a really common misconception and honestly the way a lot of beginner guides are written doesn't help. The cycle is driven by ammonia — either from a small pinch of fish food left to decompose, a few drops of pure ammonia, or a hardy fish like a zebra danio doing the work naturally. Without it, there's nothing for the bacteria to eat and nothing to cycle. The good news is your tank isn't ruined — the substrate and filter media are still there ready to go. For a 120 litre community tank, what I'd suggest is using a bacterial starter product alongside a proper ammonia source this time. Do you still have the filter media from the original run, or has it dried out?

That's actually good news — a filter that's been running continuously, even in an empty tank, can still hold some surface area for bacteria to colonise quickly once ammonia is introduced. Here's what I'd recommend: pick up a bottle of API Quick Start. It's a live bacterial supplement that seeds your filter with the exact bacteria you need — Nitrosomonas and Nitrospira — and it's widely available and well-regarded. It won't do the cycle for you overnight, but combined with a proper ammonia source it can cut your cycling time from six weeks down to two or three. Dose it into the filter, add a measured ammonia source — pure ammonia from a hardware store or a product like Dr Tim's Ammonium Chloride works perfectly — and then test every couple of days with a liquid test kit. You'll see ammonia rise, then nitrite, then both drop as nitrate climbs. That's your cycle completing.

Yes, dose it straight into the filter or pour it near the intake — you want it flowing through the media immediately. Keep the filter running the whole time, maintain the water temperature between 24–26°C if you can (bacteria are sluggish in cold water), and keep the ammonia level around 2–4 ppm as your food source. With existing wet media and API Quick Start, most people see a full cycle in two to four weeks. When you get a zero ammonia reading, zero nitrite, and nitrate is climbing, you're done. Do a 30–40% water change to bring nitrate down before adding fish, and stock gradually — tetras first, corydoras once the tank is settled. Don't add everyone at once or you'll spike the ammonia again.

You were closer than you think — the tank just needed that missing ingredient. Stick with the liquid test kit readings rather than strips if you can, they're far more accurate and will give you a clear picture of where you are each day. Once your corydoras and tetras are in a properly cycled 120 litre tank they'll do brilliantly. Good luck with it — feel free to come back if the readings confuse you at any point.