A
Honestly, after thinking back... It was my 2nd time. The first time took all 4 of my Vittatus, but all the Tinfoils and Plecos were fine.
Now I know what the problem is. I never thought the Neutral Regulator would be so strong or would hurt the water quality Thanks for the advice ?
Also, if the temperature was not really 30C but much higher, that will also contribute to decrease oxygen levels (warmer water holds less oxygen). Besides the aquarium thermometer, I also like to spot check the tank temperature with a hand held infrared temperature gun. (the temp gun is useful for other things too)