Ok, yes, I think you should water more often and adjust the amounts accordingly depending on your watering plan. What will obviously be happening is the water is transpiring away like whoa! So the 300 ml you are adding is probably being reduced by a significant amount due to the plant trying to keep cool as best as possible. You will prob find that your'e nutes may then become more concentrated, any idea of your e.c.? probably academic anyway, it will have gone up from what you are aiming at. And yes your'e right that's why it is slow to grow, it is expending all it's energy in trying to keep cool! Under the circumstances you might need to up the water amount to compensate for what you are losing. Still, that is one thing, the heat is the problem. Obvious question, you haven't got some where else to put them have you! Nah, stupid question, you would have done that by now. There is some benefit in making sure there is plenty through flow amongst the plants foliage. Any fan leaves at the bottom, that may be looking a bit peaky could be removed to facilitate better airflow. But don't go mad, remember the fan leaves are the lungs of the plant, and if you go to far, then it will be detrimental to the plant, rather than helping. Actually, the next part will probably be your best option, and your fan leaves can be spared. The fan you are using, you could set one up, or the same one so it is blowing over some cooled water, if you keep a supply of freezer blocks, the type used for camping and hiking, stick them into the cold water, and that should help immensley, both temp and humidity in fact. Just change the blocks every so often, depends on water temp, you will be able to figure out when the water starts to rise in temp again.
No problem once you get the set up right, it will be a lot easier, just a matter of tweaking here and there, I'm fortunate I have a cellar, although having said that, bigger space, more tweaks needed! Still, I suppose that's all part of the challenge and fun of it. 
But that is just my weird sense of humour!
Actually you have it backwards. Mites love and seek out a low humidity enviroment. The first form of treatment is to increase humidity to make their time in the room uncomfortable. Another treatment is to spray with straight water...why?...because mites hate moisture.At least ya' ain't gota' worry about mites with that low a humidity%.
What are you growing in? dirt or water ?
Do they perk when the lights are off? in your dark cycle?
What are your dark cycle temps?
Those are the most important in flowering
........never one answer is one mans saviour,ha ha,aint that the truth???dont chop me up and feed me to the poor,but i thought higher temps reverse mites,got to stop thinking out loud,might explain all the broken bones???????/
