I am not sure where the sand dollar came from... I'm willing to bet the previous owner found him on the beach... I can ask him if you like.
All the sites say that the sand dollars need a ton of sand to survive and might be bad for the sandbed since it can eat all the organisms in there, but so far he seems to be doing a fine job and thriving quite well in a 34g cube keeping the sand white and me happy.
In regards to the heat... 84 is too high for a unit running LEDs unless the ambient temperature in the house is 80+ which can't be comfortable to live in to begin with.
My 34g cube runs at 84-85 with mhi running for the day and found out if I open the feeding trap and sump lid the temp runs fairly constant 79-81 at the end of the day with the lights running for 9 hours.
I did get a chiller for the unit for 150$ but it's not in use ATM, waiting for the summer and then I'll run it.
Also if the tank is near a heat vent, you might want to move it to a diff location with less heat blasting directly on to it.
The tank is next to the wall where the heating pipes pass. Its my fault but moving it is not an option. The fan works great. Although I need to fill my reservoir more often. I need to do some research for some good sand sifters. My YWG is not doing its job and I am not sure if I get a diamond goby they will be territorial.