86 is a bit high, and could definately be adding to your fishes stress level. BTW, keeping 2 CBS(coral-banded shrimp) in one tank will almost always result in the loss of one or both, as they will not accept another CBS in the same tank, and will fight to the death. To reduce your temperature, try simply removing your glass tops from your tank and leaving the top open, if your lights will permit this(i.e. will sit on the edges of your tank without risking falling in). If this is not enough, you can put a fan blowing across your water's surface as well. Computer fans work well, and can be mounted easily on the back of the tank or in the hood. Your temperature could only be a factor in your problems, however, as a 6-week old tank may not have the biocapacity to handle all your fish. Have you invested in a test kit of your own? If not, you should very highly consider this. Many LFS will suggest ideas that get them sales, even(and sometimes especially) if it gets them sales, and telling you your water was okay to begin stocking could have been a lie to get you to buy fish, and even in hopes that they would die so you would come back and buy more. Even when the levels zero out and it truly is ready to begin stocking, you should add fish very slowly, IMO 1 fish per week, at the most. You have to allow time for the filtering bacteria to adjust to the increased bioload. Also, be sure you don't overfeed, as this will very highly increase waste product in the water and affect water quality. If you still have surviving emerald crabs, hermits, and snails, I would think the chances of a mantis would be fairly slim. How much rock do you have in your tank? HTH.
