From what I have found in my isopod research online, there are ones that eat meat, and ones that eat algae. The algae eating ones are ok, but if you find them scavenging on meat based foods, then you might have a problem isopod. The meat scavengers in theory will grow untill they are capable of attacking the fish, and can eventually lead to death.
Take the isopods, and put them in with a small piece fo shrimp and se if they go nuts for it. If they do, then you probably have the bad kind.
I have been trying to identify some ispods in my LR tank, and they rolled up the same as the one in your pic, but to me that doesnt look like a ball, more like a half ball. When I think ball, I think of the "rolley polley" bugs we used to pay with as kids. If anyone can confirm that pictured isopod is in fact "rolling into a ball", or if its a half ball and therefore means its a bad one.
Peace