High Nitrate


I have a 20  gal. tank with a few plants (not doing great but surviving) and a few fish 
(12 small, 3 cory cats, & 2 midium angels) which seem to be doing fine.  NH3 and NO2 are 
not present, NO3 is 200 ppm (that's as high as the test registers).  I am using an UGF 
with 2" of gravel, 150 powerhead, and an external box filter (with carbon).  After 
changing water (25%) the NO3 is still at 200 ppm.  I have been overfeeding like most 
beginners but have cut back tremendously.  What do I need to do to lower NO3?  I suspect 
that I will need to empty the tank and clean the gravel (sweeping apparently isn't 
enough).  Is there an easier way?

Also, I purchased some "aquatic plant food" from a local pet/fish store (probably a bad 
idea).  The tablets contain Nitrogen, Phosphoric acid and Potash.  When I use the 
tablets as directed it raises the NH3 from 0 to 2-3 ppm.  Is this nornal when adding 

One more question while I am asking.  Where can I find an iron test kit to test the iron 
in my water?

Thanks for helping a beginner!
Kelvin Twigg