Once you get above 70 you would have to switch how you combat pythium. I think you said you add something every so often, im assuming a corrosive to break down cell walls. Once you get into the 70's your biological growth in the res will outpace your ability to combat it with corrosives. Basically you will have to switch over to more of an aquaponics system, where you use bacteria to combat pythium. Similar to lakes and what not, vs the swimming pool method. In hydro we run live or sterile systems. Your currently running sterile with your additive. Sterile systems like temps between 66-70, lots of people use chillers. Now i run my hydro in a live system. I dont do full res changes, I dont add corrosives or enzymes, nothing. . just some bacteria. My bacteria wont work if corrosives are added.
I run warmer water temps because you get faster growth. Im assuming it speeds up the plants metabolism. If you do a side by side with 68f plants and 73f plants, same genetics, same everything, well its drastically noticeable how much faster they grow. But. . its highly debated lol. . so when we say your fine there, its because theres one hell of a learning curve if you want to really push them. So much false info out there too. Not many run warm water in hydro.