For freshwater tanks most poeple use a water-conditioner that makes the water more suitable for the fishes. It also binds harmful 'ingredients'.
Are you using a water-condtioner as well in your octopus tank?
RO (reverse osmosis) takes out most impurities and chlorine, however, tap water also often carries suspended metals. The DI (deionize) sand attracts the small suspensions. Commercial seawater salt adds back the minerals needed to correct for the deionization.