Joeman has some great advice. A lot of people run UV only when they have a problem that UV can fix (like he said, an algae outbreak). It is very helpful at those times.
Welcome! That is a neat little crab, I was curious and googled - I came up with this site - which has an impressive list of crabs and what they eat. Haven't seen your exact crab, but I will keep looking.
Could be a gorilla crab: http://www.wetwebmedia.com/swcrabfaqs3.htm
Which I guess is the family Xanthid - which I guess will eat fish. I would keep an eye on him. I had a bad experience with a killer shrimp (although rare, but I did have a shrimp kill a fish and another shrimp :( )
Welcome to the site. you have gotten some great advice so I will not muddy the waters at this time. good luck and have fun. The aqua equalizer is not a proven piece of equipment at present. I have run one for the past year and cant say it does anything helpful except my oxygen levels seem to be about .2 of a point higher. (keep it simple).