the datasheet is saying accuracy of this device is +-3 deg C what does that mean ?
+/- 3 degrees Celsius means just that. Whatever you measure could be off by as much as 3 degrees in either direction. If the fish cannot tolerate a deviation of 3 degrees above or below their nominal temp then you need to get some sealed DS18B20 ONEWIRE
16-bit precision sensors and use those or buy them in the TO-92 case package, solder the wires and hotglue or epoxy the leads to
waterproof them. My choice would be the factory made sealed probes. I don't know why you would think of using an LM60 when the poor little critters lives are at stake. What were you thinking ? XD