Franco - it's going to be equally nonstandard either way. If you say "There is less fish", then "is" disagrees with "fish". If you say "There are less fish", then "are" disagrees with "less", which requires a verb in the singular.
One rule I found that can help is that you use "less" with a word if you would normally use "much", and you use "fewer" if you would normally use "many". "There is much fish" doesn't sound quite right; "There are many fish" sounds better. Therefore, one would say "There are fewer fish". (Of course, if you're a non-native speaker, this rule might not always help, but it probably can't hurt...)
By the way, one particular case comes up all the time: grocery stores usually have an express lane for people with something like "12 items or less", but it should be "12 items or fewer". (One would have "many items", not "much items".) Grammarians and the like have been complaining about this for years, but you still see that phrase all the time...
- Kef
One rule I found that can help is that you use "less" with a word if you would normally use "much", and you use "fewer" if you would normally use "many". "There is much fish" doesn't sound quite right; "There are many fish" sounds better. Therefore, one would say "There are fewer fish". (Of course, if you're a non-native speaker, this rule might not always help, but it probably can't hurt...)
By the way, one particular case comes up all the time: grocery stores usually have an express lane for people with something like "12 items or less", but it should be "12 items or fewer". (One would have "many items", not "much items".) Grammarians and the like have been complaining about this for years, but you still see that phrase all the time...
- Kef