fresh water nitrites

my mom has a 20gal fresh water tank with 2 med angels 6 zebra danio and 2 gourami's and i know she has a few live plants in there also she said everything looks healthy but she tested the water yesterday and her nitrites were high so she did a 50% water change yesterday and tested this morning and nitrites were still high... i told her i thought the tank was kinda overloaded but what can she do to get the nitrites down?
thanks

 

Test the source water for nitrates also . Clean the gravel and filters and change filter media more often and then you can get a denitrater also. But most times its over feeding that causes the problem.. And the single most common issue I have ever seen is people just feed their tanks to death.. Cut out 1/2 of whats being fed and 1/2 as often then get it cleaned up..And make sure the new water you are adding is also NO3 free ..

 

Thanks! I relayed the info to her...