it should really say," a by-product of cactus can be used to purify......"
I don't think growing them in the recirculating system will do much
an organic filtering aide IS an improvement
most "flocc" producing products in water treatment are aluminum based

http://phys.org/news/2016-03-cactus-purifies-contaminated-aquaculture.html

Desert cactus purifies contaminated water for aquaculture, drinking and more
March 13, 2016

I wonder how this compares to using ground up Moringa seeds?