I am new to goldfish, it is swimming on surface like gasping for air every 3 minutes or so. So, I did 40% water change, using drinking water just now. Should I do something else?

There is 24/7 filter and fan, though, I think it wasn't because of lack of oxygen, or is it? Should I buy bigger tank?

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    can you please edit the latest reading of your watertest into your question ammonia,nitrite,ph.what is the temperature of the water,is the tank properly cycled fishlore.com/NitrogenCycle.htm Sep 30 '20 at 9:05
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    You wrote in your last question, that the siamese algae eater is lost in the tank, and that it was the only fish. Was it the only fish? If there was a goldfish with it, then this change the answers... And if your siamese is missing, do you let the goldfish in the same tank? Or is it a different one? Sep 30 '20 at 9:40
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    It would be a one time -pay if you for example buy 50 test stripes for around 13 euros, then you could do 50 tests... testing helps a lot, because the most "toxic" things you could not see. The water would be clear, but your fish are at risk,,, especially if you have such small tanks, then little amounts of toxic substances will cause a big effect Sep 30 '20 at 9:48
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    It takes 30 days to cycle a tank, your goldfish is suffering from ammonia poisoning.
    – Jestep
    Sep 30 '20 at 12:04
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    In your other post you said that the tank volume is 10 gallons, goldfish need at least 20 gallons for a single goldfish and an additional 10 gallons for each goldfish beyond that, these fish are generally known for being dirty and generating a lot of waste, you and the fish will benefit from a bigger tank.
    – lila
    Sep 30 '20 at 12:45