Hello,
There are a few reasons why this isn't a clever idea at all:
1. Marimo Balls are a species of filamentous algae (Aegagropila linnaei) and is closely related to cladophora. So it's ironic and even laughable that someone would decide that putting cladophora in the tank can somehow cure other forms of algae...this may be the least clever idea in the history of aquatic horticulture. This is like using BBA thinking that it will prevent BGA.
2. Reason #1 is only slightly less clever than reason #2 in that nitrates don't cause algae and are not harmful in the first place. Another irony of course is that this algae, and any other plant, will feed on the ammonia (which is the real toxin) but you don't need a Marimo Ball for that. Just plant any plant in the tank and it will do the same job -- and will look nicer. As a matter of fact, if you have a planted tank it is strongly advised to add nitrate to the tank, not remove it.
3. Bringing up the rear in the competition for least clever idea is the concept that any plant can effectively compete with algae. There is no competition between plants and algae because algae require hundreds or thousands of times LESS nutrients and CO2 to grow than any plant requires.
Having said all that, in an unplanted tank, any addition of submersed aquatic plant life will add value by removing toxins and by subsequently oxygenating the water as a byproduct, so from that perspective it would be a good thing. The balls don't require any aquascaping or maintenance and they certainly look better than other forms of algae which do exactly the same thing. But the balls certainly won't stop other algal forms from growing in the tank. That's an illusion.
Cheers,
