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Carlos,
A large water change is going to have a more immediate effect, but smaller more regular changes water changes will have the same long term result. For example, 10% weekly, 20% biweekly and 40% four weekly have the same effect over a long period, but the 40% four weekly would result in larger variation in water parameters with each water change.
I would go for the one large one followed up by smaller ones until the problem clears up. If it doesn't appear to be getting any better, you may have to move the fish and treat with antibiotics in a treatment tank.
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ATJ
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