They're called Cirolanid Isopods and they look like this:
Size can vary a lot from 1/4" to the largest I've seen being between 3/4" to 1". They can be latched onto the fish at night or scurrying around on the sand, glass, under the rocks or on it. Had a traumatic experience where I finally decided to put in fish, the first being a small pair of clownfish the smaller of which was tumbling in the water that first night and when I looked closer, there were a ton of these isopods sucking its blood and once netted, had to use tweezers to smash the isopods before pulling off because these suckers were really latched onto the fish... clownfish did not make it...
Just decided that after a month of hunting for isopods, I was too tired and wouldn't sleep sound so i took out all the rock, cleaned the tank but used some of the biologically sound water to seed dry rock, which helped.