The NFPA 70E Standard details multiple levels of Hazard Risk Categories (HRC) [see table 1.1], also called Levels. The greater the risk: the higher the HRC. Arc Rating is the factor that indicates which HRC is necessary, and can be measured in either ATPV or EBT.
The optimal pH range for brook trout appears to be 6.5-8.0,with a tolerance range of 4.0-9.5.Brook trout occur in waters with a wide range of alkalinity and specificconductance, although high alkalinity and high specific conductance usuallyincrease brook trout production (Cooper and Scherer 1967).
Q. I have a 30-gallon (114- liter) aquarium that houses three fancy goldfish and one Siamese fighting fish. The aquarium has been set up for a little over two years. The pH is 6.1, which I know is a low reading. I have read that this can be harmful to the fish and the biological (undergravel) filter in my aquarium.. The pH of your aquarium is right at the lower survival boundary for both the goldfish and the betta. As you say, it also inhibits the proper functioning of your undergravel biological filter. Although the fish have evidently adapted to the low pH, this is not a healthy situation.