[i have crabs]

calgary uses alot of silicate in the tap water which is a big factor in diatoms, other than that there is a high tds so plenty o gunk in there, using zeo probably counters the alge by using up the bad stuff but im just guessin
i used tap water for years and battled algae and cyano non stop, within weeks of ro/di water it stopped and never happened again,
in a fish only tank i wouldnt stress to much but in a reef i thinks is a bad choice