This could very well be a deficiency prob. You said that you stopped giving cal mag. Why? I don't have any experience with the nutes you are using so I can't really help there but your EC is kinda low for week 6 of bloom. You also need to take into account that the tap water you are using has minerals in it. So the ppm's from the tap plus the ppm's from your nutes is not enough for your plant anymore. Tap water never worked for me while utilizing hydro but that was me. Usually most people use RO water when going hydro. Also a Ph of 5.4-5.5 is really where you want it to be during flowering. A Ph of 6.1 will only decrease the amount of K your plant will intake. 6.1 is on the higher end of good for vegging period. The lower end of 5-6 is where you want to be for flowering.

Maybe someone who is currently doing hydro will jump in......

Have any pics to go with these probs?