I didn't specify the nitrate issue is with my cycling 20g.They are only small fish (1 inch long) and quite narrow.
------------------
If you have high nitrates, do a 75% water change and gravel clean the substrate every day until the levels are low.
Make sure any new water is free of chlorine/ chloramine before you add it to the tank.
Check your tap water for nitrates.
When you gravel clean the substrate, drain the water into a white bucket and let the water settle for a few minutes. Then use a torch to look for eggs on the bottom of the bucket.