Ammonia spike after adding fish too fast

You've hit one of the most common post-cycle mistakes — adding too many fish at once overwhelms the beneficial bacteria colony, which simply isn't large enough yet to handle the sudden bioload increase. The good news is your tank did cycle, so you have a foundation to work with. What are your current ammonia and nitrite readings, and how many fish did you add in one go?

Those readings aren't catastrophic but they're definitely stressful for your fish and will climb if you don't act now. The bacteria colony in a fishless cycle calibrates to the ammonia source you used — switching to a much larger live bioload shocks it. Your immediate priority is detoxifying the ammonia and nitrite that's already in the water without crashing your cycle further. What dechlorinator are you currently using, and have you done any water changes since you noticed the spike?

The basic dechlorinator is doing its job removing chlorine, but it's not doing anything about the ammonia already in your tank. Switch to Seachem Prime immediately — it's a concentrated conditioner that doesn't just dechlorinate, it also detoxifies ammonia and nitrite for 24 to 48 hours by converting them into a form that's harmless to fish but still processable by your beneficial bacteria. That's the critical difference here. Dose at 5ml per 200 litres as a detoxifier, or you can safely double-dose in an emergency like this. Use it every 48 hours until your readings drop back to zero.

Both, working together. Prime buys you 48-hour windows of safety, but water changes physically remove the ammonia rather than just neutralising it — that's important because your bacteria need to grow into the new bioload, and reducing the total ammonia concentration helps them catch up faster. Do a 30% water change every two days, dose Prime straight into the tank before adding the new water, and test daily. Within a week to ten days your bacteria numbers should have expanded enough to handle 12 fish in a 60 litre tank, which is a reasonable stocking level. Avoid feeding heavily during this period — every bit of uneaten food adds to the ammonia load right now.

You can add a bottle of Seachem Stability alongside the Prime — it's a bacterial supplement that introduces live nitrifying bacteria directly into your filter and substrate. It won't fix the problem overnight, but dosed daily for a week it genuinely helps accelerate the colony expansion. Keep your filter media in the tank and never rinse it under tap water during this period — chlorine kills the bacteria you're trying to grow. The combination of daily Stability doses, every-other-day water changes with Prime, and light feeding is the fastest safe route back to a stable tank.