My understanding, which is still forming, is anything to far below 66 degrees will start to slow growth and you're between 62 and 64 I think. You'll have a damn hard time getting bacteria to grow in there though which is a good thing. Not to mention all the DO you'll have because of it as well.
pithium will still grow in 64 degrees it just grows slower,The key is more air than you can afford to listen to.Think of creek in the mountains,Lots of Disolved Oxy and cooler water,I use aquarium waterfall filters in my top off res and they stay algae free because of the constant water flow.They sit at room temp.
In an environment with more organic material you need to cool the water but it does not stop pythium,I can drop my water temps to 64 and without some kind of benny or chlorine i will get pythium,Been there done that.
It so cold because , my grow room stands on concrete floor and the outside temperature in room is now 17 c . I did not isolate with rock wool the bottom of grow room .
The water bucket is 40 L running with also a small aquarium pump to create more water flow.
Bought a 200 W heater for aquarium, with idea that if i will have a bigger bucket not to buy more power full heater, but the problem was it was to power full and was heating the water till 26 c , and it also did broke.
When the water is at 20 c i can definitely see faster grow, yesterday a fan did broke and the water als odid heat up to 21 c , and the grow started so fast .
Had also a problem as i think one plant had a calcium problem , you can see in picture.