hi guys i have alway wanted red clawed crabs or a blue lobster but i have not been able to due to my fish being small. I have just got a new 30l tank and i am wondering if i could have a blue lobster in it ?
No, it is way too small even for one on it's own. They grow up to 20 cms, and wikipedia in finnish says that it needs at least 130 liter tank.
The size of the bottom area is more important than actual liters, though, so a bit smaller could do too if the shape is good, but there is no way it could live happily in a tank that small. Eventually it would have problems even to turn around!
One of the macro shrimp species may be ok in a tank that size - you mention red claws and I think they get around 2-3 inches iirc, so that might just about be suitable, however you wouldn't really be able to have more than one. I have a macro shrimp in my 240L and he definitely uses the whole tank, so a tank as small as 30L may be a little unfair.