For example: Under 40 is safe, but too close for comfort IMO..so I try and keep it at 20 at least. Mine read 10 last test.
LOL 40 I wish try 100 and not coming down.
In the last 5 weeks i have done a 10 gal, then another 10 gal, then a 20 gal along with the removal of half of my substrate, then...