ph run off coming out really high

I'm using mapito in 1meter high tubes. 3 plants per tube. Each plant is started in a 3" rockwall cube.

I'm phing my feeds to around 5.5 - 5.8 cos I noticed the ph keeps coming out high but it still 7.1 when I check the run off. I'm soaking my mapito prior to use in 5.5 ph water and even letting it drain over night, checking ph and bringing it down to 5.5 again and so forth until it comes out no higher than 5.8 but every time I feed now the run off still comes out at 7.1

WTF?

Also, I've got 7 days to get these to grow at least 5" to go into flower and they aren't growing much at all. They're under a 250w metal halide 24/7. I manually feed them hydro nutes. plant magic veg a+b, superthrive, green fuse root,

Any recommendations?

I'm gonna have to set up multiple drip systems I think. This passive hydro just isn't working very well. I've put off going full hydro due to having to have so many reservoirs and pumps etc due to perpetually harvesting but it's gotta be done.

Anyone know how the hell I could plug one of these into my hailea air pumps http://www.ebay.co.uk/itm/281124555056?var=580204848589&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649


the opening for it is massive. Not like the normal air tubing size. More like garden hose size.

come on guys?