Depending on the fish you have usually all fish can survive for a few days in the tank, only very sensitive fish will have problems after a few hours or so...so depending on how sensitive your fish is you can simply purchase a battery operated air pump to keep air in your tank...
- If the power goes out heres what I would do if I had no back-up or anything
- Change 5% of the water every couple of hours
- Usually power isnt out for more then 24hours so your fish will get oxygen from the fresh water
- Normally the temp only goes down a few degrees so no worries there
If you DO have very sensitive fish, buy a back-up power source and all the proper equipment for it but if you have fish such as guppies, platys, mollys, danios, plecs, cories, and more simple fish, there isnt much to worry about