Well slow growth can be caused by many things but judging from the conditions you listed above the only thing i can think is toxic build up on your media. This is also unlikely as if this is new media toxic build up would not have had sufficent time to occur. The right thing to flush with is something to break the bond between the various calcerous / binder materials. I use Florakleen for just this reason. Bleach's active ingredient is chlorine. Chlorine is a disinfectant and so is useless for this application. The book is great but I think we covered the bases here. I would flush with 25% nute concentration and see if there is any improvement. Also a photo would be helpful.

Herbus