if you dont want co2 i would think you would need to replace the light fixture with something less intense. you can get some work lights at home depot or lowes that use cfls. they are relatively inexpensive. then you could either sell what you have or keep it in case you change your mind about staying low tech. i think the light is your biggest issue right now, though.
you could try adding more plants to use up that light. you could also try giving the photoperiod a break: 4 hours on, 3 hours off, 3 hours on, something like that. liquid carbon melts some plants, which might be why the wendtii leaves are melting.
surely someone else will have better advice. i'm not exactly experienced enough to give advice, just trying to help. i have had a t5ho on a 29 gall and had all kinds of problems until i got the pressurized co2 and ferts. i still have to give the photoperiod a break for a couple of hours a day to prevent the BBA from taking over. i also spot treat with h2o2 when doing water changes but i would rather not do that on a regular basis. i am finally getting it taken care of. i think the h2o2 and the break in the photoperiod have been most useful as the addition of co2 alone did not help much (although my plants loved it).
I have gone to a place far below up above and in between the middle.