I have almost the same water as ufo 55 is listing. MY GH and KH run around 12 German degrees which is in the same basic range as his. My pH tends to run around 7.8 and my tap water is treated with choramines so right after a water change I see the effect of breaking the chloramine bond and releasing ammonia at about 1 ppm from my tap water. My usual water change routine is to limit new water to less than 25% of the total so I never see more than 0.25 ppm of ammonia in the tank itself, and that is very short lived. What I have found is that almost all of my tanks do best with straight tap water. Some of the very sensitive fish that I sometimes keep get a mix of tap water and RO from my own RO unit, but most get straight tap water. The end result is that even some fish reputed to like low pH low mineral content water do fine in my tanks. The albino bristle nose plecos are even breeding regularly in water that people will tell you is completely unsuitable for them. On the other hand, my water makes typical common livebearers iron clad in my tanks. Plants can get all of their trace mineral needs right from my tap water with no additions needed.
I have exactly 2 of my 26 tanks that ever get a mix of RO and tap water. One is a tank where I am trying to breed Corydoras hastatus, which get a 3/1 mix of RO. Their tank pH is still 7.8 but the mineral content is much lower than in my tap water, at only about 100 ppm of TDS. My RO water has a pH near neutral and a mineral content of about 17 ppm of TDS, which equates to less than 1 degree of GH and KH, far purer than the water ufo 550 is getting from his pet shop.
The best advice I can offer, for the ongoing fish problems, is to stop using RO in any way. Instead, do a few small water changes over time using tap water. Let's say 10% water changes each time. After you have done 5 or 6 water changes that small every 2 days, your fish will be fairly well acclimated to your tap water mineral content and will tolerate much larger water changes with ease. Whenever I find a water problem in one of my tanks, every few months when you have as many tanks as I do, I do 90% or larger water changes. Right after a water change like that, all fish in the tank look far better than they did before the change. If necessary, based on testing results, I will repeat that water change daily. After a few days, my tanks often look like cycled tanks because I clone the mature media that I have in old established tanks, so I need not keep up that huge water change regime for very long.
When I set up a new tank, I never lose many fish. Instead I take very aggressive measures, with very large water changes, to keep ammonia and nitrites under control. I find that an auction, like the one I will attend tomorrow, often leaves me with the need to quickly bring on a new tank to house the new fish. As long as I remember that fish in the wild see new water daily, remember much of what we keep come from running streams, I find that my new fish are quite responsive to water changes. Rather than stress out about what the pH is going to do in a tank, I focus on the contaminants in my tank. No fish in the wild must put up with much ammonia or nitrite since the small stream they live in replaces its water several times each day. Rain does change the pH dramatically and also changes mineral content quite a bit in very short time periods yet the fish thrive in that small stream. What the fish tolerate easily is a pH change. They can also adjust to changing water mineral content, but not quite as quickly.