Guys i have a cycled 45 gallon tank that currently has 10 tiger barbs and a RTS in it. 0 ammonia 0 nitrites and 5nitrates. I was wondering if i could add a Blue Lobster as the LFS has some in stock.
The blue crays are infamous for killing and eating fish. I have one in a 55 with my turtle and some briggs and a few spixis. I have only recently noticed that he has been snacking on my spixis. If you provide enough hiding places maybe, but be prepared to rehome the cray.
It really depends on the actual species the LFS has. "Blue Lobsters" are usually either P. Alleni or Cherax Quads. Alleni are highly aggressive and will attack fish and other tank mates (often for no good reason). Quads are a lot more docile and are more easily kept with fish and other tank mates, however, adults will be close to 11in when full grown (some closer to 14in depending on the breeding background). Generally, if you can provide enough space, hiding spots, and food you can avoid casualties for the most part but it's more than likely to eventually occur.
If you're set on having crayfish, pick a more docile species or a dwarf species. Cajun Dwarfs, CPOs, CP sp Shocko, Montezumaes... they stay under 2in roughly and don't bother fish, they do nibble on aquatic plants though. And if you want to try a larger species, marbled and appalachian brooks are very docile for the most part. There's still a potential for a lost fish here and there, but it's significantly less than with most other species.