A few months ago I purchased a Hammer's Cobalt Blue lobster -- Procambarus sp. and not Cherax sp. -- from LiveAquaria.com and that boy has never been any trouble to the plants or the fishes. Only time I saw him do anything aggressive was to wave his claw at a dollar that was trying to take his wafer.
He's in my 4 year old son's 20 gallon planted aquarium and living with 3 silver dollars, 2 tiger barbs, 1 Blue Dwarf Gourami, and a pl*co. These are semi-aggressive fish and they don't bother him and he doesn't bother them. I don't know how he'd be in a peaceful tank. He's moulted once (boy, THAT was cool!) and he comes to the front of the tank every night (like clockwork) for his wafer.
We've never had a problem with Mr. Cleaner (my son named him and I think that name rocks). Never had a problem with the plants and the dollars either until recently
I put a couple of new plants in the tank and I guess the dollars liked the taste. ROFL! Gone bye-bye
Anyhoodle, the Cobalt Blue only gets to 5" in an aquarium and he's an awesome color.
I DO have a cherax sp. in another tank and that boy is VISCIOUS! He's in there with 2 guppies and a gourami. If you even get close to the tank he's got his claws up in the air and ready to go a few rounds. My daughter (she's nine and it's her lobster) named him "Slifer the Sky Lobster" but I think his name should be Rocky Balboa
Slifer pulls up the plants and throws the bogwood around. He has dismantled the filter tube more than once, too. I'm waiting for him to mangle the airline tubing next
I'd give a huge thumbs up to Procambarus sp. in a semi-agressive tank, possible risk in a peaceful one.
Roan