Weird fish deaths

[plantedtankman]

Recently 3 of my Congo Tetras have strangely died all of the sudden. My Nitrate level is at 0 and my Nitrite level is at 0. The ammonia level is at 0 and the pH is 6.8. My tank is planted and the fish show no signs of disease. I wonder why my Congo Tetras are dying.

 

[Khuligirl93]

What size and for how long has your tank been set up? 0 nitrates is uncommon for a cycled tank.

 

[plantedtankman]

What size and for how long has your tank been set up? 0 nitrates is uncommon for a cycled tank.

For about 5 months actually. My tank is a 30 gallon.

 

[plantedtankman]

Actually the Nitate level is above 0. The yellow looks darker than the yellow on the card.