| Penny,
From your description it sounds like you have Sphaeromatid isopods (pictures provided by seahorsedreams above). Cirolanid isopods can be predatory or parasitic, but will not eat algae, so you should be in the clear. Unless you see some of them attached to your fish, then you probably just have a population of Sphaeromatids.
Their population will probably wax and wane and some other tank inhabitants may eat them. Sphaeromatid isopods are generally considered a beneficial inhabitant and we should do everything we can to encourage beneficial inhabitants as it makes the already weak food web in our tanks stronger.
Brian | 
