I've had some bad luck with some recently purchased 4 apistogramma cacatuids orange flash , they all died within the week. The tank is 75 galleons has only 4 Otto's in it . This is the second batch I've lost and the last unless i can figure out what I'm doing wrong. The fish where all bought online from reputable people. The PH is 6.8 , KH 40ppm, GH 25ppm, nitrite o, nitrate 20ppm and temperature 81 degrees. I do 20 % weekly water changes have a hard time keeping nitrate below 20ppm . Thanks for any recommendations.