how to have golden fish???

geogie

My friend gave me four golden fish and a very big tank. But after two months, thes golden fish all died. We think maybe they were infected some virus or bacteria because we found that their tails are all damaged. Here I ask some having experience why? We usually have them in the temperature about 22-25C and feen them once a day, usually at night.
3 V5 z4 m* B: Q2 A% I  j+ p& WThank you very much.
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Xiaoxiao