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Abstract
Dissolved organic carbon in culture water and the light absorption due to the dissolved organic substances in a closed recirculation aquarium increase with the cumulative amount of food, especially in a linear correlation within the relatively low accumulation. The absorption was extremely high at the far-ultraviolet region and the spectrum exhibited no peak. The salt concentration of the culture water showed no effect on the light absorption. Aquarium water used for long cultivation was highly fluorescent. Ultrafiltration of the culture water revealed that more than half of the total dissolved organic substances had a molecular weight of the order of 10 4. The elements C, N and P in the organic substances accumulated in a culture water accounted for only 3·2% of the amount added to the aquarium as food even in the most extreme case. Four experimental cultures with different intervals of washing filter sands revealed that the growth of fish and the accumulation level of nitrite were arranged in the order of frequency of washing, while the organic substances were in the reverse order. Accumulation of organic substances may be a cause which leads to the suppression of growth of fish cultured in closed recirculation systems.
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