I use ProMix when I want more water retention. It's fine particles, but they don't compress too quickly, so they still let the roots get air. I find I get better foliage production and color in it vs. a bark-based mix. If there's a hydroponics store near you, they can probably special order it.

Well, 511 is a very well drained mix. But you can reduce pine bark and increase peat moss (make it 50/50) and eliminate perlite and replace it with Vermiculite. While perlite does not absorb any water, Vermiculite is highly absorbant. You might also want to try adding some Floor Dri (DE) which also retains a lot of water (almost 100% of its weight)

3-3-1 ( 3 parts pine fine, no nuggets, 3 part peat moss, 1 part vermiculite) This is just a conceptualiztion of a moisture retentive mix.

I am sure the experts will chime in.