While macroalgae can and does filter out heavy metals, it does not know how to deal with chloramine and chlorine.
We want water for our tanks with a 0TDS reading. TDS is Total Dissolved Solids measured in Parts Per Million (ppm) (or occasionally in ppb). If your municipal water supply puts chloramine into your water, the macroalgae will not remove it from the water column. Some salt mixes do have a little bit of tap water conditioner in them, but it dissipates over time and is not meant to keep removing it from top off water.
In my area, I have a tap water reading of 500ppm TDS. That is extremely high. I also have chloramine in my local water supply - which is a good thing and protects me from bad bacteria in the water, but a bad thing for my (former) marine tank. In order for me to get a TDS reading of 0ppm and protect my RO membrane from damage by chlorine/chloramine/ammonia, I have to use two catalytic carbon filters, and a 5 micron sediment filter. I also have to have a membrane that has a high rejection rate - like a 50gpd membrane (98% rejection) so that the TDS coming from the membrane that is leftover won't be too high and cause my DI resin to be exhausted as quickly.
TDS as high as 20ppm wrecked my last tank from bad top off water, and I even had macroalgae growing.
