get a PH kit and test the water. its really easy.
the others may have suggestions on what PH works best for particular species. In general 6.5-7.0 is ok for most fish. Best is if all species belong to one parameter, but it's flexible in most cases. Tap water PH depends on your area.
if it's too low, you can add things like coral rock to the filtration to bring the PH up or add things like wood or (forgot the name) filter element to bring down PH.
amonia/nitrite/nitrate are controlled also via filtration and just as important. But these are not as easily adjusted. quality of filter system + water changes to control here.
may be your area has high chlorine/flouride or other additive content. buy a water filter, cheaper over bottled water over time.
what filter system is in use currently? imo keeping fish is more about keeping water than the fish. You'll be suprised how much more vibrant in color and activity inhabitants display when parameters are good. it sounds like you will re-populate the tank with significantly more fish than before. If so, may be a good idea to hone in on what killed the pygo piranhas. i've found when they are all healthy and well fed, own species killing does not occur, but when one individual is weak and unable to fend for itself, it becomes food. so perhaps in this particular case, the water was the core culprit.
btw, Cannibolism is innate in this species... but they do better in larger numbers if you ever try to keep them again. the more, the better (given enough space/filtration). in a 150, i'd say 8 is a good conservative number.
birchirs are cool and from a patient-fish-layman perspective, very striking. I would as others have suggested fill the mid-upper water levels with some colorful species too large for birchirs to consume, and that won't outgrow the 150.
a tank is a great thing to have in the workplace.