the water you used likely has at least trace amounts of dissolved metals such as copper, from piping, and other dissolved metals as well. Reverse Osmosis removes 99% of it. You can get it for around $0.30 a gallon. If you continue to use tap water, or poorly filtered water, the dissolved metals do not evaporate with the water and WILL become a concentrate at some point. Most likely your existing fish would adapt to it, but it will bring phosphates, algaes, and other small problems that can lead to bigger problems. I took my cans back today and filled my tank with 60 gallons of ro water. Added salt, and mixed it up and ran filter, added sand, and when the temp is right, I am putting my live rock in! Protein skimmer needlessly running... Just trust everyone when we say use ro water. Since you are still cycling, you could do some hardy water changes, like 5 gallons every couple days using ro water, this will minimize the metals you have in the water now. This will increase your cycle process though, as you should let it cycle longer if you do changes like that. Good luck with whatever you do.