Setting up a new aquarium is quite exciting, but the one common problem which most beginners face is the sudden death of fish within a few days or weeks. That is pretty common, and the good thing is that it is quite easy to prevent. In this guide, I will explain everything in simple words so that everyone can understand how to keep fish safe in a new tank.
1. Give Your Aquarium Time to Cycle
The biggest cause of fish dying in a new tank is due to the water not being ready. A new aquarium does not have any good bacteria that clean harmful waste.
When fish excrete feces or uneaten food starts to decompose it produces:
Ammonia (very hazardous)
Nitrite (also harmful)
Nitrate: safe in small amounts
In the new tank, ammonia rises very quickly and becomes toxic, causing the fish to die suddenly.
How to fix this
Set up the tank
Add water conditioner
Run the filter for 7–10 days
Add fish slowly and not all at once
If you add fish on day 1, they get ammonia poisoning.
2. Use a Good Water Conditioner
Tap water has chlorine and heavy metals. Those things can burn the fish gills and cause death.
Always add conditioner first in the tank before adding water. This makes the water safe in just a few seconds.
3. Add Only a Few Fish at the Beginning
In a new tank, bacteria are very low. If you add too many fish, the waste becomes too much.
This creates ammonia spikes and fish die.
Rule for beginners
Add 2–3 fish first, wait 1 week, then add more slowly.
4. Don't Overfeed Your Fish
New fish owners feed too much. Extra food sinks and starts rotting.
Rotten food = ammonia = fish death.
How much to feed
Feed only that much which the fish finish in 30 seconds.
5. Install a Strong Filter
Your filter is the heart of your aquarium. It cleans the water and keeps oxygen flowing.
A weak filter causes:
Dirty water
Bad odour
Low oxygen
Dead fish
Choose a filter by tank size
30L tank → 300 LPH filter
60L tank → 600 LPH filter
100L tank → 800–1000 LPH filter
Never run an aquarium without a filter.
6. Properly Change Water
Many people change all of the water. This destroys all the beneficial bacteria.
Never do a 100% water change.
Do only:
20–30% water change weekly
Add water conditioner
Use water at room temperature
7. Maintain Stable Temperature
Sudden temperature changes shock the fish.
Ideal aquarium temperatures:
Goldfish → 20–24°C
Guppy, Molly, Platy → 24-27°C
Betta → 26–28°C
Always use a thermometer and heater, if needed.
8. Don't Add Fish Immediately After Cleaning the Tank
If you wash everything with soap or fully clean the tank, the bacteria die. Without bacteria, ammonia rises fast.
After fully cleaning:
Run the filter for 24 hours.
Add conditioner.
Then add fish.
9. Avoid Mixing Aggressive Fish
Some fish kill or injure others.
Example:
Betta with guppies
Goldfish with tetras
Oscars with small fish
Before purchasing fish, make sure they will be compatible.
9. Avoid Mixing Aggressive Fish
Some fish kill or injure others.
Example:
Betta with guppies
Goldfish with tetras
Oscars with small fish
Check the compatibility of the fish before purchasing.
10. Quarantine New Fish
New fish sometimes bring diseases.
Keep them in a small separate tank for 3–5 days, if at all possible.
This helps prevent:
Fungal infections
White spot
Parasites
11. Oxygen is very important.
Low oxygen causes sudden death. You’ll see fish gasping at the surface.
To fix this:
Add an airstone
Keep the filter outlet splashing the water
Avoid overheating
12. Test Your Water
If fish are dying, always test:
ammonia
Nitrite
Nitrate
pH
Safe levels are:
Ammonia → 0
Nitrite → 0
Nitrate → below 40
pH → 6.5–7.5 for most fish
Change 30% water if ammonia or nitrite is high.
13. Don't Buy Sick or Weak Fish
Before purchasing, look for:
Clear eyes
Active swimming
No white dots
No torn fins
Sick fish easily die in new tanks.
14. Keep the Tank Away From Heat & Sun
Direct sunlight grows algae and heats the water.
This causes stress and a drop in oxygen.
Place the aquarium in a cool, shaded corner.
15. Be Patient
A new tank needs 2–4 weeks to become stable.
If you stay patient and follow the steps, your fish will live long and stay healthy.
Conclusion
Most fish die in the new aquarium because of uncycled water, ammonia buildup, and simple mistakes made by beginners, such as overfeeding or too many fish being added to the aquarium.
Sudden fish death can be completely avoided by proper preparation of the tank, using a good filter, feeding correctly, and maintaining stable water quality.
With patience and care, it will make your aquarium a safe and healthy place for all your fish.