How to increase yield ???

Hi Everyone!!!
My friend grows indoors! during flowering Using 2/1000w Hps. in 5 gallon buckets, using Pro Mix HP, with 8 1/1/2 inch holes for drainage. 4 in the bottom and 4 1/3 up. he puts them in when they are aprox 24 inches high. Temp is kept at 78 degrees under 12/12. with Co2 at 1500ppm. PH at 6.5. He uses Fox Farm 3 stage {Open Sesame, Beastie Bloomz and Cha Ching!! }using recomended dosages on Fox Farm website! Watering twice a week. 1/3 gallon each time. alternating {feed}{ water/epsom salt}. But the Yields are Extremely LOW! Like 1oz per! Then he increased Dosage to 2X amount at each stage and it increased yield and potency! His question is How much of a dosage is optimum? and How much is Negative?? And also what other addatives you would recomened to increase to maximize the overall Yield, Quality, Potency, and Aroma?
All opinions appreciated!!!!
Thanx
 
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