This experiment evaluated petunia (Petunia ×hybrida ‘Celebrity’) growth response to amending a commercial potting mix (F3B) with different amounts of dewatered aquaculture effluent (AE) and fertigating with a water-soluble, inorganic fertilizer or municipal water. The experiment was a completely randomized 2 × 5 factorial design with eight single-pot replications per treatment. At 39 days after planting (DAP), a significant (P ≤ 0.05) substrate and water interaction existed for petunia growth index (GI), bloom count (BC), fresh weight (FW), and dry weight (DW). The 100% F3B, 5% AE, and 10% AE substrates benefited with a significantly (P ≤ 0.05) greater BC, FW, and DW when fertigation was used; however, the water source had no effect on petunia BC, FW, or DW for levels ≥25% AE. Fertigating substrates amended with increasing levels of AE did not improve petunia growth. Conversely, when applying municipal water plant, FW and DW were the greatest using 25% AE.