Originally posted by OrionGirl
Is the tank cycled? Did you acclimate the critters to your water conditions? If not, this is more likely the problem, especially if the tank isn't cycled.
If you mix RO water, about 50/50 with your tapwater, you'll lower the KH significantly. If you start using this for water changes, the changeover will be gradual for the fish. However--I'd prefer acclimating the fish. Changing the water is not the easiest thing out there, and the variety of snakeoil products--things like pH-Down or ProperpH--make it seem like a casual undertaking. Those products tend to make pH unstable, and the resulting yo-yoing is much worse for your fish than being at a stable but high value.