my goldfishes stopped eating

Question
i have an (adult hand sized)goldfish for 7 years and a 14 in. catfish in a 30 gal. tank. Usually, changing water does not affect them in anyway, cause they are strong fish. However, since their last water change, both have stopped eating and do not respond to people anymore. Even the catfish(who goes berserk if he senses movement) is inactive. We usually put lukewarm water for them since its winter, but its changed back to cold water. Is it a problem with the cold water temperature? Its often gets cold in the house, so the water is even colder. What's the lowest temperature they should sustain, before adding a heater to the tank?

Answer
Hello,

Coldwater fish do not need a heater, but just like any fish they need a stable water temperature that will not fluctuate. The sudden cold snap is likely what is causing your fish to be ill. Adding lukewarm water to the aquarium could further the shock. The temperature should remain steady at a mark between 68 and 72 F. If your water is anywhere below 68, or even if it is simply fluctuating greatly throughout the day, you should definitly invest in a heater.

I hope this helps.

Regards,
Stephanie