Foreground plants usually don't grow fast, the fastest growing plants are mostly stem plants. You could try glossostigma, it is supposed to be quite the weed. Dwarf sag starts out short and will grow taller when it gets crowded.
Try to find wisteria, water sprite, maybe hornwort. Brazilian pennywort is a fast grower for me. Some people really like elodea and Cabomba, haven't had them for ages and don't remember how fast they grew. The sword plant you have in the middle looks like a shorter type, you could get Amazon sword, that will grow quite fast. Rotala rotundifolia and Ludwigia repens are easy stems that are good for filling out new tanks. Watch out, some Rotala and Ludwigia species are more difficult plants.
I would spread out the crypts more by dividing what you have into bunches about the size of the crypt on the right near the white rock. Foreground plants are great but get the tank filled in with fast growers for right now. I love pennywort and wisteria is terrific stuff as well.