Common Aquarium Mistakes (and How to Avoid Them)
Most new fish keepers lose their first fish to a handful of avoidable mistakes. Here are the 10 most common ones — and the simple fix for each.
1.Overfeeding your fish
Uneaten food rots and spikes ammonia — the #1 cause of water problems and fish loss in new tanks.
Fix: Feed only what your fish eat in 1–2 minutes. A tiny pinch, once or twice a day. Skip a feed once a week.
2.Doing a full water change
Emptying the tank wipes out the good bacteria that keep the water safe, and shocks your fish.
Fix: Change only 20–25% of the water every 1–2 weeks. Top up evaporation with treated water in between.
3.Cleaning the filter under tap water
Chlorine in tap water kills the beneficial bacteria living in your filter sponge.
Fix: Rinse the sponge in a bucket of old tank water you've just siphoned out — never under the tap.
4.Adding fish before the tank has cycled
A new tank has no good bacteria yet, so fish waste turns straight into toxic ammonia.
Fix: Run the tank for 2–6 weeks with the filter on before adding fish. Use a live bacteria starter to speed it up, and test the water first.
5.Adding too many fish at once
A sudden overload of waste overwhelms the filter and causes an ammonia spike that kills fish.
Fix: Start with 2–3 hardy fish, wait a week, test the water, then add a few more gradually.
6.Keeping a goldfish in a bowl
Goldfish produce huge amounts of waste and grow large — a bowl can't support a filter or enough water volume.
Fix: Give a goldfish 60L+ per fish, a strong filter, and cold water. No heater needed.
7.Placing the tank in direct sunlight
Sunlight causes algae blooms and swings the temperature, stressing your fish.
Fix: Place the tank away from windows. Use a timer for the LED light — 8–10 hours a day is plenty.
8.Keeping male bettas together
Male bettas are territorial and will fight to the death if housed with another male.
Fix: Keep one male betta alone, or in a large, heavily planted community with peaceful, non-fin-nipping fish.
9.Ignoring the water temperature
Tropical fish need steady 24–27°C. Sudden drops stress the immune system and invite disease.
Fix: Use an aquarium heater with a thermostat and check the temperature daily. Replace a faulty heater immediately.
10.Buying fish without researching them
Some fish need schools, some are aggressive, and some grow far too big for a starter tank.
Fix: Research each species before buying — adult size, tank size, temperament, and which fish they're compatible with.
The golden rule
Almost every mistake on this list comes down to one thing: patience. Cycle the tank before adding fish, add fish slowly, feed sparingly, and change water little and often. Get those four right and you're ahead of most beginners.
More than just fish
My First Pet helps with every first pet — explore our other beginner guides:
Recommended on Amazon
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Water Test Kit
Test ammonia, nitrite, nitrate and pH to keep your fish safe.
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Water Conditioner
Removes chlorine and heavy metals — essential for every water change.
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Live Bacteria Starter
Bottle of good bacteria to speed up tank cycling — use when setting up.
Check price on Amazon
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