I understand that weed advice is like assholes. Everybodys got one but my post advice comes from people that I have been able to verify are knowledgeable, experienced growers. Many are licensed operators with staff botanists so I give their advice more creedence than I would someone that I have not been able to verify.
The temp advice I gave comes from a RDWC expert out of Canada that is a licensed operator. We were discussing how water temperature affects oxygen levels in the feed solution and someone in the discussion mentioned that they were taught that water temps in RDWC should be on the cooler side, around 68°, because cooler water will hold more oxygen molecules. My Canadian RDWC guy commented that, while theoretically this is true, oxygen levels in water will dissipate quickly trying to equalize with the existing air pressure in the air so oxygenating, while it does help, isn't quite as effective as some think. He actually tested this and his results were as he stated. Yes the water had a higher concentration of dissolved O² but it was around 2% - 3% higher than normal standing water.
He also commented on how applying too much air into the chamber will affect EC and pH levels. This is why he says that while oxygenating your water with an airstone is a good idea it's not providing as much O² as, say, fogponics where the roots are suspended in air and being misted constantly rather than the roots sitting in water relying on the O² levels in the water. He said that he too was taught that cooler waters meant more oxygenation but he said he experimented with higher res temps and saw no detrimental side effects in regard to plant performance. He reported that he found increased growth with higher air and root temps. His only comment was that with the higher res and room temps you wanted to keep an eye on crown rot but he said that he's only experienced it once and he felt that it came from another source rather than res temps.
I'm a dirt farmer with just enough DWC grows under my belt to be dangerous but the advice applies across the board. Laboratories have confirmed that the ideal leaf temps for photosynthesis are 77°/25C in almost all plant species. The only time that would change was if the plant originated in an extreme weather area where the genetics have adjusted to the climate but for cannabis all the landrace cultivars that have been discovered have been found in a fairly tight grow zone so, for the most part, cannabis knowledge can be used almost across the board when growing it.
If you are still in doubt look to CO² growers. They typically grow in temps hovering from 85° to 95°. Higher temps faster growth as long as the rest of the stats are in range.
