I agree with the above. You must not only acclimate the temperature but, also the water parameters. The drip system works best and can take a couple of hours to acclimate new critters, fish AND inverts. Many invertebrates are even more sensative than fish. Unlike freshwatre, there is more than temperature to equalize in marine tanks.
Never directly add salt to the tank. Get some RO water and mix the salt in there. Get the salinity right and add slowly to the tank. Better yet, let the water "age" a few days before adding.
The clawn is probably not going to survive and now look for ich. many times when a fish gets so stress, there can be an ich outbreak......another emergeny article by itself.
Undergravel filters were the way to go 20-30 years ago and are OK for freshwater but, can cause a severe nightmare in a marine tank if you ever lose power. Put a battery back-up on the pump for the filter like you would buy for your computer. If your power goes out, the filter bed, which is aerobic, will quickly die and go anaerobic. This can kill the entire tank in 24 hours. If you never lose power, the filter will NOT hurt your tank if it has a separate power supply. Most marine tanks rely on the live rock and live sand as the main filtration media. We then add protein skimmers, refugia or wet/dry filters and sometimes hang on the back filters for added water filtration.
Mike