flippi

Did another 50% water change. How long will I have high nitrite before I should start to panic?

I have a "new" 2nd hand tank setup. Tank is 170L; with 120-140L of actual water content. I have a Aquais500. New gravel and some plants. My first week all tested ok. I have 10 guppies, 7 mollies and 3 smallish catfish. On the 2nd week one guppy had 19 babies, they are now one week old. I may have done a bit of overfeeding. Since the weekend I had a maxed out Nitrite and nitrate test. I did a 10 to 15% water change each week. Re tested the weekend and still have a 5ppm Nitrate reading. Did a 50% water change on Monday, and still have a close to 5ppm nitrite test. (API kit bright purple) PH and Ammonia is ok. Fish seem to be happy and the babies all alive and growing fast. I am using Prime to condition the water. My question is why after a 50 % water change do i still see a high Nitrite reading? How long will it be this high? My next steps will be to do a gravel vacuum and remove some plastic plants. I'm getting a second smaller tank for the babies. Both females are due to give birth again sometime soon.