I've heard one may feed an animal on shabbos but may not feed a fish,being that tthey could live a day without food, is that accurate?
Yalkut Yosef 1:373:15 says that one may feed fish in his fishtank on Shabbos.
מותר ליתן אוכל לדגים שבאקווריום בשבת
Rabbi Chaim Tabasky also says it is permitted.
This is only permitted for pet fish. One may not feed fish in a lake or ocean on Shabbos as they are not his responsibility. OU.org - Feeding the fish