Two experiments were conducted to evaluate the effects of two different sizes of extruded pellets and moist pellet or dough type diets on growth and water quality in olive flounder, Paralichthys olivaceus and Japanese eel, Anguilla japonica. In the first experiment, olive flounder averaging 5.0±0.2g (mean±SD) were fed one of the 3 experimental diets; one moist pellet type diet being used by local farmers and commercially available two different sizes of EPs (EP1-3mm, EP2-5mm). After 8weeks of the feeding trial, weight gain (WG), specific growth rate (SGR), feed efficiency (FE) and protein efficiency ratio (PER) of olive flounder fed EP2 diet were significantly higher than those of fish fed EP1 and MP diet. COD, NH3, turbidity and total phosphorous from tanks of fish fed EP1 and EP2 were significantly lower than those from tanks of fish fed MP diet. In the second experiment, Japanese eel averaging 21.3±1.0g (mean±SD) were fed one of the 3 experimental diets; one dough type diet being used by local farmers and commercially available two different sizes of EPs (EP1-2mm, EP2-3mm). After 10weeks of the feeding trial, WG, SGR, FE and PER of Japanese eel fed EP2 diet were significantly higher than those of fish fed EP1 and dough diet. COD, NO2, NO3, NH3, turbidity and total phosphorous from tanks of fish fed EP1 and EP2 were significantly lower than those from tanks of fish fed dough diet. These results indicated that fish fed EP2(5mm) had the better growth parameters and the better water quality parameters than did fish fed MP in juvenile olive flounder averaging 5.0±0.2g (mean±SD). And fish fed EP2(3mm) had the better growth parameters and the better water quality parameters than did fish fed dough diet in juvenile Japanese eel averaging 21.3±1.0g (mean±SD).