Probiotics are beneficial microbes that are being studied more in aquaculture, as they offer an environmentally sustainable alternative to antibiotics while improving fish growth and feed efficiency. The present study investigated the effects of water-applied commercial Lactobacillus rhamnosus on the growth performance, feed efficiency, and survival rate of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) fingerlings. The experiment ran for 60 days of culture from September 07 to November 05, 2025, and was conducted at the Multi-Species hatchery, Bongao, Tawi-Tawi, Philippines. A total of 450 O. niloticus fingerlings were randomly distributed in 15 tanks. Fish were fed proportionally at 10% of their average body weight (ABW) twice daily. The experiment used a completely randomized design which included three replications and five treatments that tested different L. rhamnosus concentrations through control: 0 mg L-1; T1: 1 mg L-1; T2: 2 mg L-1; T3: 3 mg L-1; T4: 4 mg L-1. Data were analyzed using JASP Version 20, with normality and homogeneity tested. The results show that specific growth rate (SGR), weight gain (WG), and total length gain (TLG) were significantly increased at 4 mg L-1 of L. rhamnosus (p < 0.05). Whereas feed efficiency and survival rate did not differ significantly across the treatments (p > 0.05). A quadratic regression estimated that the optimum concentration level for maximizing SGR was 3.63 mg L-1. Water-applied L. rhamnosus improved tilapia growth, which may indicate its potential as a water additive in aquaculture systems.
