Regenerative and integrated multi-trophic aquaculture (IMTA) are increasingly positioned as practical, nature-based pathways for aligning aquaculture expansion with ecosystem restoration, nutrient management, and blue carbon development across Southeast Asia. Realising this potential at scale depends on species-level evidence that extractive organisms can reliably convert dissolved aquaculture waste into recoverable, value-added biomass. This study contributes such evidence for the tropical red seaweed Gracilaria changii, a species already central to Malaysian pond-based regenerative aquaculture initiatives, when co-cultured with Nile tilapia (Oreochromis niloticus) in brackish water. Two complementary laboratory experiments, examined G. changii's dual function as a biological nutrient filter and as a route for carbon incorporation into harvestable biomass: a 7-day polyculture trial assessing the effect of fish feeding on water quality and seaweed growth and tissue stoichiometry, and a 35-day recirculating trial comparing three seaweed stocking densities against seaweed-only and fish- only controls. Seaweed growth and nitrogen assimilation rose markedly in the presence of fed fish, while the highest stocking density delivered substantial reductions in total ammoniacal nitrogen, nitrate, and orthophosphate, alongside improved fish growth performance— demonstrating the kind of nutrient-recycling synergy that underpins IMTA system design. Seaweed tissue carbon content remained stable across treatments, confirming that nutrient uptake translated into new, carbon-containing biomass rather than transient physiological change. These results support G. changii's role as an effective, low-input nutrient biofilter suited to abandoned or underutilised brackish-water ponds, reinforcing its relevance to ongoing regenerative aquaculture, bioremediation, and biodiversity-driven blue economy efforts in the region. At the same time, the findings underscore an important distinction increasingly raised in this research space: measuring biomass carbon capture is not equivalent to demonstrating permanent carbon sequestration. Future research should quantify dry-biomass production, establish complete carbon budgets, assess air–water CO₂ exchange, and determine the fate and storage permanence of harvested biomass. Addressing these evidence gaps is essential to validate regenerative and blue-carbon claims and to support the development of seaweed-based IMTA as a credible, scalable strategy for climate-resilient and nature-positive aquaculture.