Believing that "treated" tap water was a good idea. I couldn't figure out why I had a 90% mortality rate. Finally, after 2 years of frustration , I bought a cheap counter top ro/di unit and haven't lost a fish since. Oh the $ I could've saved. BTW, I was amazed at the difference in sg readings between my refractometer and the hydrometer. Buying a refractometer and water filter is the two biggest things I recommend to new aquarists. Some folks say refractometers are over-rated, I say I wouldn't use anything else, now.