temperature of water

im new to hydro what temp should my res be kept at, sorry i know this has been asked a million times i just cant find it

as a general rule low 70s (22-24c) is good. Plus or minus a couple degrees fahrenheit

The colder the water, the more dissolved oxygen it can hold. Water temps in the range of 58-65 degrees Fahrenheit hold a greater amount of DO than temps in the 70's. If you can afford one, a reservoir chiller can be had to reduce temps to the lower range. Some people have had moderate success with using frozen bottles of water to cool their tanks, but this can be rather materially and time intensive versus a chiller.

i tend to keep mine between 20-21c

thank you kindly gentleman your advice is greatly appreciated