A couple points to make... 1) not all isopods are created equal and 2) culture them outside the tank if you want to use them as feeders.
Native isopods (such as
Porcellio scaber), while certainly cheaper, definately are not the better feeder. They make fine take custodians, but that is about it. For a slightly "prettier" (when you rarely see them in the tank) you can try the orange isopods, which are mearly an orange population of
Porcellio scaber. These isos are rather large and hard shelled, and just don't make good feeders. The young are tiny and make good frog food, but are hard to seperate from mom, and aren't produced in significant numbers. Small tropical feeder isopods tend to have size and shell softness on their side (meaning you can feed out most life stages), with more frogs taking them as food. They can also be used as a cleaning crew, but they might get eaten more
If you want to use them as any significant source of food, culture them on their own outside the tank (and preferably save yourself some effort and use the smaller, softer species). They are slower to reproduce than some of the most popular feeders (then again, most critters are!) with 3+ month generation times. The ones in the tank will probably not produce as much, especially when the population stablizes, and will be hiding away from the frogs, so they will only be a once in a blue moon treat.