Deaths right after new fish or plants
This looks like: ammonia / nitrite spike from extra bioload — or toxins from a dead fish tucked away
Why this happens
New fish add waste instantly. If the filter wasn’t ready, ammonia and nitrite can climb overnight. One dead newcomer behind décor rotting fast can poison a small tank the same way.
What to do right now
- Test ammonia and nitrite now — if you don’t have a kit, borrow one or buy liquid tests today.
- Find and remove any dead fish or uneaten meaty food stuck behind wood or plants.
- Do a 30% water change with temperature-matched, dechlorinated water if ammonia or nitrite reads above zero.
- Ease feeding to once daily light meals until both read 0 ppm for 48 hours straight.
- If the tank was already full, use the stocking calculator before adding more.
What not to do
- Don’t add “starter bacteria” blindly instead of testing — know your numbers first.
- Don’t pour in medications that don’t match the symptom, you can exhaust fish further.
Stop it happening again
Add fish a few at a time rather than filling the tank at once, quarantine new arrivals where you can, and test ammonia and nitrite a day after adding anything.
If that doesn’t fix it
If parameters stay clean but losses continue, inspect for aggression at lights-out and consider isolated quarantine for next purchases.
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.