Answer:
Your answer would be that there is no error in the sentence.
Step-by-step explanation:
The sentence above does not contain any mistake. As you can see, the event took place in the past. There is phrase indicating this: the last time. This NP, the last time, sets the event in the past. Consequently, the verb phrases should also be in the past tense (visited, said, were).
Thhere is no reason why "park" or "you" would be incorrect. The park is the place where the event took place and "you" refers to one of the people that visited it previously. What is more, the existential phrase is not a mistake either because the sentence states that there were lots of fish. The noun might have the same form in singular and plural but the quantifier indicates that this is the plural form. Finally, the PP "in the pond" is also correct because is the place where the fish are.