Honestly I'm not sure. I found a site that has an MP3 of how it's pronounced but I can't listen because my speakers are unhooked atm
http://www.forvo.com/word/loy_krathong/
Not knowing how to pronounce it right I'd guess like Loy (roy)... I'm probably completely off base here though lol. Just thought it was a neat name. Plus I generally don't ever talk to my fish and say their names. It's usually "Hi guys! Time for food." Or other stuff like that ;) I just saw your guy and thought festival. he's kinda grizzled like he has points of light all over him.