Abstract: The evolution of cognitive abilities is a central topic in comparative cognition, with particular attention given to whether trade-offs exist among different cognitive domains. In this study, we systematically measured the performance of 96 individuals of crucian carp (Carassius auratus) in three cognitive domains—spatial learning, quantity judgment, and inhibitory control—to investigate individual differences and their interrelationships. Spatial learning was assessed using a T-maze task, quantity judgment using a 1 vs. 2spontaneous quantity preference task, and inhibitory control using a “visible but inaccessible” food task. The results showed that task validity tests confirmed all three cognitive abilities were effectively elicited under our paradigms. Individual differences in the three cognitive abilities exhibited a gradient pattern: spatial learning showed the greatest variation, followed by quantity judgment, with inhibitory control showing the least. Simple correlation analysis revealed that spatial learning ability was positively correlated with quantity judgment performance and negatively correlated with inhibitory control ability, while quantity judgment performance and inhibitory control ability showed no significant correlation. However, partial correlation analyses (controlling for body condition and feeding motivation) and censored data sensitivity analyses indicated that these significant correlations no longer held. These findings provide an important qualification to the cognitive trade-off hypothesis: at the level of individual variation in natural populations, superficial associations among cognitive domains may be substantially confounded by non-cognitive factors (e.g., body condition and feeding motivation), rather than directly reflecting competition for cognitive resources.