<p>The colonization of the biofouling diatom <i>Fragilariopsis oceanica</i> on the tank surface, in culture water, and on macroalgal biomass can lead to a decrease in the quantity and quality of biomass yield in land-based seaweed cultivation systems. This study aimed to assess the influence of various GeO₂ concentrations, including (LDI) 0.1&#xa0;mg L⁻¹; (MDI) 0.25&#xa0;mg L⁻¹; and (HDI) 0.5&#xa0;mg L⁻¹, on the <i>F. oceanica</i> load and its concomitant effect on the growth, biomass, and photosynthetic pigment production of <i>Ulva ohnoi</i> under varying light intensities, of 67, 134, and 201 µmol photons m^-2&#xa0;s^-1; culture media, MP1, 3NP, and CF; and photoperiods of 12 L:12 D, 16 L:8 D, 24 L:0 D, followed by validation in a scaled-up production system with a 0.5 m³ capacity through optimized parameters. Among the tested combinations, DGR_max (12.73 ± 1.2% day⁻¹), length-based EGR_max (3.78 ± 0.13% day⁻¹), and photosynthetic pigments_max (Chl <i>a</i> – 68.64 ± 1.2; Chl <i>b</i> – 45.29 ± 1.8; total Chl – 140.44 ± 1.0&#xa0;mg&#xa0;g⁻¹ fw) were observed. Furthermore, protein_max (66.36 ± 1.86%), and carbohydrate_max (53.62 ± 0.18%) were observed at 201.1&#xa0;µmol photons m⁻²&#xa0;s⁻¹ under MDI and LDI, respectively. The load of diatoms was significantly reduced in the urea:DAP-fed media and a similar trend was also observed in the scaled-up experiments (in the 0.5 m³ tanks). These optimization strategies will have a significant positive influence on elite quality biomass production of <i>Ulva</i> through controlling diatom biofouling, and this novel innovation will aid sustainable and efficient <i>Ulva</i> cultivation for increased productivity in tanks and photobioreactors and can be scaled up for commercial applications. Additionally, these optimal parameters for <i>Ulva</i> cultivation will increase functional food development, drive global demand, and increase coastal livelihoods.</p> Graphical Abstract <p></p>