The Prime could be causing the false test results with ammonia. Most test kits (except Seachem's!) will show water treated with Prime as having a trace of ammonia. This has to do with the chemicals involved to dechlorinate the water.
Adding any chemicals to RO
water is kind of undoing the whole thing you were trying to achieve - pure water. The carbon block before the RO
membrane will eliminate the chlorine in the water before it even hits the membrane. And if your fish are healthy, there's no need to any type of "slime coat" products. Give your fish good clean water, and let them be fish... they'll be fine.
My guess is that if you stop with the Prime (highly recommended!), your ammonia "problems" will go away. The amount of ammonia Seachem is talking about in their salt mix should be unmeasurable using the hobbiest test kits we all use - regardless of brand.