If you want to make your own CO2 you can get a water bottle with some water in it. Then add equal amounts of yeast and sugar. The yeast will metabolize the sugars and produce CO2 as a byproduct. If the bottle stops bubbling give it a good shake and it should start again. If it completely stops bubbling then you need more yeast or sugar. Keep in mind that you would probably want your fans off when CO2 is being generated so that you dont lose it before your plants can use it. There is also a CO2 tablet you can buy that you drop in water (I think its water) and it releases CO2. I am not sure on the effectiveness of soda water spraying...but one thing to keep in mind is that too much water on your leaves that is constantly evaporating can actually dry the leaves even more. It sounds a little counter intuitive, but evaporation can draw moisture from what it is being evaporated from.
As far as what you can do to help them get better...reduce heat (obviously), CO2, ensure that your PH and nutrients are at ideal levels. Damaged portions of the leaves is something you are going to have to live with. You cant make dead tissue start growing again. If some of the lower leaves are pretty much gone then you may want to consider cutting them off.
I cant really tell what type of system you are using there, but you would want to check the temperature of your water, too. Water that is too warm can also do bad things to the half of your plant below the ground.
