Using tap water is a roll of the dice. I set up my first octopus tank with tap water and raised a bimac just fine. Later, I hatched out and reared several O mercatoris and kept a pet vulgaris. Just three miles down the road, and set up my tanks with tap water again. The first octopus lived a few months, the second a few hours. I tried adjusting the filter. I got a shipment of cuttles from the NRCC and THEY died. I tried other things. I tested for everything you can test for- ph nitro compounds, heavy metals, copper....it all came up nil. I added a toxin sponge compound to the filter and changed out the all-activated carbon canister filter. Instead of sacrificing another cephalopod to Jimbo's Cephalopod Gas Chamber, I added cheaper, more mundane creatures in cluding some starfish that IMHO are way more delicate than cephalopods....and they thrived. Satisified with this fix, I ordered three baby bimacs from the now-defunct cephsource. They all died inside of a week.
Then I bought and installed a cheap kent marine RO unit, and generated the water necessary to do a total tank teardown. After going through all the amazing hassle of completely redoing a tank with all fresh mixed water and re-cycling it, I ordered another cephalopod....and watched it carefully for day after day after day after day.....
It's been fine since. With basic RO units going for less than a hundred bucks (
http://www.drsfostersmith.com/product/prod_Display.cfm?siteid=6&pCatId=4467), save yourself TONS of aggravation.
Rock on, Jimbo