Nine successive 5-month experiments which examined relationships between fertilization strategies, water quality and yields of Nile tilapia took place February 1985 through March 1990 in 16 earthen ponds at the Ayutthaya Freshwater Fisheries Center, Bang Sai, Thailand. Over the course of these experiments, randomization of treatments resulted in all ponds having different fertilization histories. Analysis of covariance was used to quantify the carry-over effects of nutrient inputs from earlier experiments on experimental errors in Experiment 9.