substrate health

I have a 29 gallon tank with alot of plants and a good number of fish.
 The tank has been very healthy, but I'm concerned about the
substrate.  I had been vaccumming it -- as much as possible without
disturbing the plants -- but there is still plenty of organic material
building up from plant waste, fish waste, and some black worms that
managed to wiggle their way down to avoid becoming dinner, then dying
anyway.  My concern is that I may be building up sulfides as there is
some odor coming from the tank and the plecos will kick up the
substrate and you can see bubbles released.  How do I best take care
of this?  Are there any natural inhabitants that I can add to a
freshwater tank that will feed on some of this substrate mess?  The
plecos eat just about everything, but the gravel is just getting too
much buildup.

TIA!