My goldfish are dying after a few weeks
I bought three goldfish from Carrefour last week and two are already dead. The water looks clear, I feed them once a day. Is there something I'm forgetting or are they just fish that are too difficult to keep?
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The real question is the size of your tank. Goldfish need way more space than people think - like a minimum of 80-100 liters for the first fish, and 40 extra liters for each one you add after that. If you've got them in a small tank, that's why they're dying, no matter how well you take care of them. It's not that they're difficult, it's that we keep them in suffocating conditions.
After that, the other crucial thing is the nitrogen cycle. Even if your water looks clean, you need to let your tank run without fish for 3-4 weeks before you add any. During that time, you plant live plants and add a source of bacteria (tank from a friend, used substrate). Those bacteria develop and turn toxic waste into less dangerous nitrates. If you put the fish in right after filling it, ammonia and nitrites poison them slowly.
For your survivor, do gradual water changes of 20-30% each week, nothing more. Get the water tested if you can to check ammonia and nitrite levels. And honestly, ease up on the food - once a day is already too much for most people. Two or three pinches that they finish completely, that's enough.
Clear-looking water doesn't mean much - it's the biological cycles that kill the fish, not visual cleanliness. Goldfish produce a lot of waste and without beneficial bacteria established in the filter (or the gravel if you have one), ammonia builds up and kills them in a few days. You really need to let the tank cycle for 2-3 weeks before adding fish, or use products that speed up the process. Once a day is fine for feeding, but without a proper nitrogen cycle you could feed zero times and it wouldn't change anything - it's really regular water changes and especially a functioning filter that save their lives.
Goldfish are actually pretty robust, so if yours are dying it's rarely because of the species itself. The ones from Carrefour are often stressed and weakened even before they get to you, which plays a big role - once they're in a new environment that isn't optimal, it finishes them off. The main problem is probably that you're doing water changes that are too drastic or too frequent trying to compensate, which creates thermal and chemical shocks. Try letting your tank run for a few weeks before adding fish, without touching it, and then do really gradual water changes - like 20% per week max.
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