I think i have an answer for your tanks with low ph! So i rented a book from the library right? Its called livebearers by wilfred l whitern.
It says:
An excess of decomposing organic matter (extra food/rotted plants) also competes with the fishes for oxygen. Not only does it deprive the fishes of oxygen, but it also causes an increase in the amount of carbon dioxide that is dissolved in the water, and this results in drastic changes in the chemisry of the water, often dropping the Ph well below the level most fish can tolerate.
It says:
An excess of decomposing organic matter (extra food/rotted plants) also competes with the fishes for oxygen. Not only does it deprive the fishes of oxygen, but it also causes an increase in the amount of carbon dioxide that is dissolved in the water, and this results in drastic changes in the chemisry of the water, often dropping the Ph well below the level most fish can tolerate.