They are found in fresh and salt water. Also, because, like Daphnia, brine shrimp and fairy shrimp, they can lay resting eggs (or cysts) when conditions get bad, they are found in temporary bodies of water. Sometimes creatures like them will "bloom" in a pond after the ice is out.

(A neat field trip in the next few weeks, in areas coming out of winter, is to find a temporary pond in a field, forest preserve, by a railroad track or even in a rutted area. Bring a gallon jar or plastic box (and a clean bucket maybe). Run a fine meshed net through the water and observe what you have. Your public library will have photo books of pond life, crustaceans and the like.

Hard shelled, they are not usually a food of first choice. Also the larger ones which appear every spring in my Daphnia cultures seem to appear not too long before the hair algae on the side of the barrels goes away. That may be a concern for you.

Those “weird free swimming brown specks” (also black in some cases) probably hitchhiked, in person or as eggs, on the Marimo Ball. Since they may also nibble said algae, I would suggest not feeding your fish for a couple of days – if there are no fry which might get eaten by the hungry guppies. Most small aquarium fish will then eat them. Maybe those crustaceans will perform the role of roughage, much like brine shrimp shells do.

You probably could gravel vac around that algae ball (which is endangered now in parts of its native Japan) and keep their population down. I prefer a biological control and like to use any legitimate food source for the fish. :)

I have tried to release a larger species of Ostrocod into tanks with algae. I’m going to have to remove any fish first though, if the algae is to be successfully grazed. ;)

You may want to draw a few out of the tank with a turkey baster and take them in a jar or rinsed medicine vial to a local biology teacher. (Call ahead.) They can more effectively identify them. You may be able to dredge up a book and hand lens and I.D. them as well.