I just use an all-purpose fertilizer once every two weeks (Miracle Grow). I'd advise against anything that's too harsh and could burn the roots. Also, if your cuttings are well-established (they look like they are), I'd take them out of the humidity tents. Excessive humidity promotes rot and slows growth.
I've attached a photo I just took of one of my plants. It has never been in a humidity tent, never misted, etc., and it's growing just fine. (It's only 7 weeks old.)
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salvia20110520.1.jpg
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