Only the new arrivals die
This looks like: new fish stress plus bacteria brought in with them — not always “your ammonia”
Why this happens
Bags carry different pH and temp. Fish white-blood-cell crash while your ammonia test still reads pretty. Sometimes the bacteria hitchhiking on new stock matter as much as parameters.
How sure are we? Very common pattern for newcomers — still test ammonia on day 1 and day 3.
What to do right now
- Float sealed bags for temp only if shop water is extreme — otherwise drip-acclimate slowly in a bucket.
- Dim lights, skip feeding new fish for 24h, offer tiny meals after that.
- Run a simple quarantine tank next time: sponge filter, heater, mature media shared from main filter squeezings.
- Match shop salinity/pH when brackish or soft-water species — look up species cards first.
- Delay adding more fish until you're confident the tank absorbed this batch's bioload.
What not to do
- Don’t pour bag water into the tank — it is not a free bonus.
If that doesn’t fix it
If deaths repeat from one supplier, diversify sources or ask about shipment dates.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.