Some people are very particular about what they put into their tanks, and are very careful about measurements and distilled water. Other people, less so. It's up to you how careful you want to be. Tap water shouldn't have any living organisms, which is why it comes chlorinated. But without a sterile environment, you'll be introducing all sorts of organisms into the mix. So, non-transparent container, or in the dark, and a bit of excel or some other sanitizer.
Iron and phosphate don't play well together, which is why you shouldn't be mixing the micros with the macros, or at least not the KH2PO4. That's also one reason to avoid tap water, as that "other stuff" almost certainly includes Fe.