Imperfect or indefinite?

Hola Stuart,

You could actually use eran or estaban. If you use "eran" you are speaking about a quality that is "inherent" to the noun: "los mariscos eran ricos y frescos", for example. But we tend to use estar with the adjective "rico" because we generally use it when we talk about something that we taste (or tasted). It is as if you take yourself back to when you were tasting it and express how good they were at that time of tasting them. It is to do with somebody's personal opinion at the time.

An inherent quality is always going to be the case and not so much a personal opinion. 

I hope this helped clarify it.

Inma