IDK if you changed your bucket setup or fixed that light leak or what you have going on... but, if you start getting shit growing in your res, it can absolutely raise your ppm.
...ask me how I know ;p
Maybe you res is in good condition... but since we don't really know, first thing is to make sure you are taking care of the basics so you can be sure no slime/algea/etc is growing in your buckets. Ensuring there are no light leaks, keeping temps as low as possible, and using either benes or sterile. I don't know if you filled those net pots up, but make sure there is 0.0% light getting into your buckets. Put a layer of panda film or cardboard or something on top of your net pots if you have too. The 16oz ice bottles do diddly fuck in 5g buckets. Use 1L bottles of ice and swap every 12-24 hours. Your 68-71* is fine. Use benes or h202/bleach... I like pond-zyme.
I see in your recent pictures, light can still shine into your buckets through the net pots. I could be waayyyy off.. but I'm gonna take a stab in a dark here... I'm going to guess 1: You have light leaks. 2: Not using any kind of benes or h202. 3: Temps are probably drifting up into low-mid 70s by the time you change the bottles. If that is the case, you will def fight with root rot and shit.