I think (but I have no proof for it) that the lack of oxygen is the reason.
The high temperature in your res is no providing too much O2.
That, and the high temperature of the water itself, are imo not ideal. Most plants (including most strains of cannabis) like 20-25 C.
If you would build a cloner with sprayers and put them on a timer (so on 1 minute, off 8 minutes) you would give your clones a lower temperature because your pump (that will also heat up the water) is not running 24/7 AND there would be more O2 available (in gaseous state) when the pump is off.
I grow clones in air temperatures of around 30 degrees. With low RV (30) and sometimes with high RV. (60)
I just use tapwater. Nothing else. I don't change it for 2 weeks.
Succes rate of 95%.
And the reason some clones die on me, is because I treat my mother plant really bad, and I sometimes cut off a branch that is not really ready to become a clone. Or that is too 'woody'.
Perhaps your clones are also too small when you cut them.
Or too much light above it. Or too little. There are many factors that can be the reason.
One thing you can also do between cloning attempts, besides thoroughly cleaning your system, is allow the components to receive some outdoor sunshine.
UV radiation exposure will kill the bad bacteria causing the slime and rot on your clones.
How does this work then?
If the UV radiation reaches your roots, you are doing something wrong anyway.
Light reaching roots is causing slime and algae.
And besides that: It is only UV-C that can kill bacteria and viruses.
UV-C in sunshine does not reach the earth. It is blocked by the ozon layer.
