What are you using as a source of ammonia? a 75g with 1-2ppm of ammonia is a lot for 1 platy. lol.
When you are doing the 50% water changes, what are you treating the water with? Prime will give a false/positive on ammonia tests, so if you are changing 50% of the water, then treating with prime, then testing a bit later or even the next day, it will show a false/positive. Maybe hold off on the water changes for a few days and then test and see what the readings are. Other alternative is to get the seachem total/free ammonia test which will test and show what the true ammonia and ammonium levels are. Ammonium is the byproduct of prime binding with the chlorine.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps