I just added 3 juvenile Cories to my 20g QT yesterday. I seeded filter sponge/biomax in my display tank for a month and used that to insta-cycle my QT a few hours before adding the Cories. Today I tested the water just to check on it and I got 0 ammonia/nitrite/nitrate. I was wondering about the 0 nitrates...is it just that the three itty bitty Cories haven't produced enough ammonia to show nitrates? Also, with just three juvies in there, how often should I do water changes? Once a week, twice a week?