Music0 min ago
goldfish
Answers
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For more on marking an answer as the "Best Answer", please visit our FAQ.You don't say if it's a tropical or coldwater set-up you have, or what fish they are. But if you go along to your local aquarists / pet shop and describe the problem (giving as miuch information as you can) - then he may be able to suggest something to help - usually this is in the form of drops put into the water.
The causes of disease can vary, from bacterial and fungal infections to poisoning from toxins that have built up in the water.
Bad water management won't help any of your fish. You must make sure you do partial water changes regularly (don't empty the whole tank in one go - aim to replace between 25 - 35% of the tank water every few weeks) and add a few drops of a tap-water conditioner to the new water. It needn't be a chore - but your fish will be so much healthier - and happier.