Gravel Vac Question

My 4' tank has been established for nearly 5 weeks now and has cycled
nicely. The water is superbly clear and the parameters AOK. But the plants
are poor, straggly things which dont seem to be growing roots properly,
dammit. The substrate is quite fine gravel. I believe my lighting is ample
and I've been adding plant fertilizer daily. Having googled the symptoms it
seems I need to increase my light time so I've now done that. But in the
course of googling I now think I may be being a bit too enthusiastic with
the gravel vac during my at least weekly 25% water changes. I've been
pushing it right to the bottom of the gravel and delighting in all the yuck
that comes up. Maybe I'm going too deep and robbing the plants of the
nutrients. Waddyathink? What advice would you give me? Thanks.