False positive nitrite test

Okay, I'm befuddled.  I set up another (!) tank three weeks ago - gravel, 
nice piece of driftwood (purchased, not found), two coconut half houses 
(de-fruited and scrubbed) and plenty of plants.  I tested the water the 
day before yesterday (before putting little Kribs in - hallelujah) and 
the nitrite level is off the scale; with a Tetra test, one drop of the 
second reagent turns the water deep, dark red.  The other kit is an A.P. 
dry-tab test - turns darker than the indicator for a level of 5 ppm.
I've done two 50% water changes and the results are the same.
There is also a low level of ammonia.

I tested other tanks to make sure the reagents haven't "gone bad" and 
the other tanks test fine.  A test of tap water shows no measurable nitrite.

My question:  could something in the coconut shells be resulting in a 
false positive test?  Or could they be the source of the nitrite?  Would 
the little bit of decaying plant matter drive the level that high?  (There 
was no filtration on the tank until Wednesday.)

Since I don't need the tank this minute, it's an interesting puzzle.