Mainly I am having this problem with my 72 gal tank which only has 4 smallish frontosa in it. I can't understand why this tank with only 4 fish very frequently reads 0.25 of nitrite, yet my 55 gal with 18 mbuna and a decent size plec is rarely positive for anything. I feel like I'm constantly changing water. Does it seem normal to always have nitrite but no ammonia? I tested my tap water and there was no nitrite in it. There are no live plants in either tank. I keep the water in the frontosa tank a few degrees warmer than the mbuna tank. Both tanks have been up and running for almost 6 months. Thanks!