Are you still using distilled water? Distilled water is essentially void of minerals, causing it to be prone to rapid pH fluctuations that can kill fish. Most important, however, is the effect that this purified water has on fish: the fish is essentially living in a very hypotonic solution, which means that the water will constantly be rushing into the body through osmosis in the gills and other exposed areas. This puts immense strain on the kidneys, which must produce copious amounts of very dilute urine to maintain a proper internal osmotic balance.
That being said, with an ammonia reading of .5 ppm, I would continue to use the distilled water with the addition of a mineral supplement (which nullifies the aforementioned problem). In fact, I'd probably use in in all the tanks unless they are cycled - prime will only detoxify ammonia into ammonium for a limited time, so if beneficial bacteria are not available to use the ammonium, the water will eventually become toxic again.
I hope this helps you out and that your fish recovers soon!
20 gallon long: 3 adult Neolamprologus similis + about 11 fry of various ages; low light planted tank
20 gallon long:2 freshwater dwarf puffers (Puff Puff and Poofer); medium-light planted tank
10 gallon: 1 male betta named Wormy; low light planted tank
10 gallon: 1 male betta named Dante; low light planted tank
2, 5.5 gallon tanks that are currently empty (I see more fish on the horizon