Two experiments were conducted to evaluate the efficacy of an oil-based substrate, The Water Cleanser? (TWC). The first experiment studied the effects two substrates of different oil composition (TWC, TWC?+), and a commercial bacterial additive (Bio-Aid) on concentrations of nitrogen and phosphorous in indoor aquaria. The second experiment studied the effects of TWC, a bacterial additive, and a combination (TWC?+?B) on concentrations of nitrogen and phosphorous, phytoplankton abundance and diversity in outdoor freshwater crayfish (Cherax cainii, Austin and Ryan, Invertebr Syst 16:357–367, 2002) tanks. In the first experiment, the concentration of TAN decreased more rapidly with Bio-Aid, whilst the maximum concentrations of NO₂-N and NO₃-N were reduced with the substrates. The concentration of orthophosphate was reduced in aquaria with TWC?+?. In the second experiment, concentrations of TAN, NO₂-N, NO₃-N and total phosphate were not significantly affected by TWC. After addition of TWC?+?B, there was a significant decrease in the concentrations of nitrate and total phosphate, and a higher abundance of phytoplankton was maintained than with other treatments. Additionally, a population Bacillus sp. was found on the substrate surface. TWC had no adverse effects on phytoplankton abundance or diversity, or C. cainii physiology, weight gain or survival. TWC and TWC?+?were effective bioremediators of eutrophic water, whilst a combination of TWC?+?B was effective in short term bioremediation and in promoting phytoplankton abundance in C. cainii tank culture.