Cultures of the blue green algae (cyanobacteria) Anabaena wisconsinense and Oscillatoria curviceps were isolated from fish farms and their antimicrobial effects were studied. Solvent extracts of exponential phase algae were screened for antimicrobial activity against different species of Gram positive (Lactobacillus sp. and Bacillus firmus) and Gram negative (Aeromonas hydrophila, Pseudomonas fluorescens and Pseudomonas anguilliseptica) bacteria and the fungi (Aspergillus niger and Saprolegnia parasitica) which were all isolated from diseased fish. Maximum antimicrobial activity was observed with ethanolic extract of O. curviceps against Lactobacillus sp. and Aeromonas hydrophilia, whereas, methanol extracts of A. wisconsinense and O. curviceps had antibacterial effects against B. firmus, A. hydrophila, P. fluorescens, P. anguilliseptica, and the fungi A. niger and S. parasitica. It was also observed that solvent extracts of A. wisconsinense when injected into Oreochromis niloticus which was already injected intraperitoneally with 2 x 10 6 CFU/ml P. anguilliseptica significantly decreased fish mortality from 50 to 12.5%.

The present study was carried out to evaluate heavy metals residues in water and fish (Oreochromis niloticus) tissues collected periodically from summer 2007 to spring 2008 of different water sources (agricultural drainage water, mixed drainage water, and fresh water), while there were no fish could be found in mixed drainage water. There were significant differences among different water sources as well as different seasons. Water samples had Fe residuals level exceeding the permissible limit in all different water sources while Pb level exceeding the permissible limit in mixed drainage water in spring season. Bioaccumulation of Fe, Zn, Cu, and Cd varied in muscle, gills, and liver tissues of Oreochromis niloticus. Generally, it could be mentioned that heavy metals concentrations in different investigated fish organs were in the following order: muscle < gills < liver. Lead concentrations were not detectable in different tissues of fish collected from different sources during different seasons. The higher values of metals were detected in fish collected from the agricultural drainage water. Moreover, there were seasonal variations had been observed, where cadmium residuals level had exceeding the internationally permissible levels in winter and autumn.

Eruption of blue-green algal blooms occurs frequently in eutrophic lakes and fish ponds, with associated unpleasant odor and horrid scums. In the present study, we conducted a pre-test experiment in 3 m(3) outdoor concrete ponds to determine the optimum concentration of aluminum sulfate (alum) required for reduction of the cyanobacterial blooms without negative effect on fish growth. As a consequence, 10 mg L(-1) alum was named as the optimum concentration that was applied in 1000 m(3) earthen fish ponds. Obtained results showed that Secchi disc values significantly increased from 10 to 24 cm after 14 days of alum application. Alum-treated ponds showed a reduction in total phytoplankton counts by 94 and 96% compared to the corresponding controls after 10 and 14 days, respectively. Abundance of blue-green algae in the treated ponds was decreased by 98% compared to the corresponding control after 14 days of alum application. Consequently, dissolved oxygen, pH, total phosphorus, orthophosphate, and chlorophyll "a" content declined significantly. Our study revealed that using 10 mg L(-1) of alum is an effective way to control cyanobacterial blooms in eutrophic waters, especially in fish ponds, without negative effect in water quality.

Mixtures of different concentrations of both nitrogen and phosphorus were used at various N/P ratios to investigate the maximum growth of Nile River Phytoplankton using bioassay studies. A ratio 4:1 N/P gave a high algal mass production and high content of chl(a) of phytoplankton. A high diversity value 1.48 was obtained during the experimental run. Bacillanophyta was the dominant group followed by chlorophyta and cyanophyta in all treatments. The growth rate reached its maximum value (1.69) after 16 days of incubation. Positive correlation was established between phytoplankton and orthophosphate, nitrate, organic nitrogen and dissolved oxygen of the Nile River water.

he purpose of this study was seeding or inoculating the Chlorella elliposoidea (Gerneck) + Scenedesmus bijuga (Turpin) as green algae at the beginning of the production season (or before the preferable time of cyanobacteria growing) to propagate and prevent the growth of harmful cyanobacteria in the Nile tilapia culture via glass aquaria.Indoor experiment was carried out in natural light using 12 glass aquaria as four groups (3 replicates each). Ten Nile tilapia fingerlings were stocked (Oreochromis niloticus) in each aquarium.The cyanobacteria were not appeared in the fish aquaria which were seeded or inoculated with Chlorella + Scenedesmus 20 x 10 3 cells ml -1 (T 3 ) at day 5 and 10. The cyanobacterial count was lower in the second treatment (T 2 ). Significant differences were observed between the cyanobacterial count on day 5 and 10 in the control aquaria and those inoculated with Chlorella + Scenedesmus sp. (p < 0.05). Higher counts of green algae were observed in aquaria inoculated with the highest dose of Chlorella + Scenedesmus sp. (T 3 ). Generally, the green algal count in control aquaria was the lowest, followed by those treated with 10 x 10 3 cells ml -1 Chlorella + Scenedesmus sp. (T 1 ). No significant differences were observed in the green algae and cyanobacteria counts on day 5 and 10 between T 2 and T 3 (P < 0.05).The present experiment showed that the presence of Chlorella + Scenedesmus sp. is sufficient to control the growth of cyanobacteria for 10 days period in the Nile tilapia culture.