Do you take the rez temp after you open it or while the pump is running ?

Just ask because 78 is still high for rez temp. With the R/O water it is perfect to grow something if warm and no chlorine or anything to kill that unwanted growth. A ciano-bacteria can set in and then you really got a problem.

Def try what was said and add something to stop it.
Also lower those temps in the rez. I love a good chiller.