Hi Jefferson, The first sentence is actually incorrect. It looks like it's meant to be in the past, so you would say, "It was unusual for them to eat fish." And then the second sentence is talking about the present.

Hola :) No es común o no los vemos a menudo comiendo pescado. ¿Entiendes? Y la manera correcta de decirlo sería: It's unusual for them to eat fish.

I agree with Jen Heavener. The first sentence seems grammatically incorrect to me. As Jen says, you can correct it by saying "It was unusual for them to eat fish. Another option is to correct it by adding the word "that": "It was unusual THAT they ate fish." The structure is like this: "It was surprising that he called me." "It's great that we were allowed to leave work early." "It's unfortunate that we have to wait." "It's strange that she hasn't replied." "Isn't it exciting that we're moving to Spain?" It may be that in some regions or dialects of English, it's okay to leave out the word "that," but I've rarely heard anyone do this in my part of the US. We always use the word "that" to connect the two ideas. "It was surprising." What was surprising? "[The fact] that he called me." "It was unusual that they ate fish" is grammatically okay, but the sentence is still a bit unclear, because it could mean either: A: It was unusual that they ATE FISH. They ate fish, and eating fish was an unusual thing to do. Most people normally don't eat fish. OR B: It was unusual that THEY ate fish. They rarely ate fish. Eating fish was an unusual thing for THEM to do. Jen Heavener's suggestion is much more clear. "It was unusual for them to eat fish" clearly means that they normally did not eat fish.

For me, the first sentence implies that at some point in the past, they ate fish instead of what ever else was being offered at that particular time. While the second would mean that generally they don't eat fish - they may tend to avoid eating fish for some reason.

The first is speaking of something that already happened, while the second is speaking generally and applies to the past, purebred or future.