Narrowing down tank bacteria

I've been feeding my fish Lactobacillus on both flakes and pellets. However, I haven't been finding any Lactobacillus in their feces. I tested the tank pH with a calibrated meter and recorded a value of 6.5 (+/- .2pH). Various species of Lactobacillus grow best at acid pH values. The tank water is within this range. The control tank had a measured pH of approximately 7.5 - 8 (measurements fluctuated with location). I inoculated an MRS plate to test growth. If there are Lactobacillus growing within the experimental tank, I wonder what they are consuming (flake particles?).

The sample in the experimental tank were Gram negative catalase negative rods that showed positive lactose fermentation (acid production) in double strength solution. They also were positive for gas production within the lactose. This colony may be Erysipelothrix rhusiopathiae which is common in fish. Although they share some characteristics, they differ significantly in length - which I'll measure tomorrow. The Lactose fermentation was weak - which also suggests it is Erysipelothrix rhusiopathiae. The Bergey manual also suggests a glucose test to help differentiate them.

Below is the Blood Agar result which suggests Erysipelothrix rhusiopathiae. (alpha hemolysis)