hrmm cool I would not think that 500 or 600 ppm is burning those big SOL girls at this age... your runty Fantasmo is another story, more like she has been runty/stunted and struggling to keep up from the beginning, and keeping her at the same ppm, well...
but the SOL leaves.... what kind of lights and how close are they? the temps and pH are good? sorry I suck at this honestly