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.
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.
Stop it happening again
Quarantine every new fish for two to three weeks, drip-acclimate slowly, and never tip shop bag water into your tank.
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.
This isn't veterinary advice - it's a directional read to help you decide your next step, based on the most common causes. We can't see your fish, so pair it with a liquid test kit and clear photos, and see an aquatic vet or a trusted local fish shop for a valuable or seriously ill fish.