While that's a small amount, any nitrite is a sign of something wrong. You said it was <1.5mg/ml and that the lowest possible value was <0.3mg/ml. That means you have nitrite and something is up with your beneficial bacteria. Like I said, ammonia spikes happen sometimes (they shouldn't in a stable tank but things happen and 5-gallons are anything but stable) but a nitrite spike means the tank is crashing or wasn't cycled to begin with. Make sure both ammonia and nitrite are 0 (or the lowest possible values) before getting more frogs.
I'm not too familiar with that filter (we don't have that brand here) but it looks like it's internal, allowing the frogs to potentially get sucked in. I would use a HOB filter (the Azoo palm filter is very popular for small tanks--and cheap) and cover the intake tube with some sort of mesh (stretched out pantyhose works!). Either that or use a simple sponge filter, keeping in mind that it'll need to be cleaned regularly. I don't have any personal recommendations for a sponge filter--they all work pretty much the same.
I'm normally an advocate for 20% water changes at least every other week, with at least 10% changes a week being more ideal. But lately I've changed my thinking a bit in regards to small aquariums. Obviously they need frequent water changes because of the fast build-up of pollutants--but at the same time, it's easier to stress the organisms by doing large water changes. So now with desktop aquariums, I change 5% twice a week. You end up changing out the same amount of water with less stress to the animals--and it's easier for you, too (making it more likely that you'll stick to the schedule)
When toxic ammonia and nitrite are concerned, though, you want to do large water changes to dilute them (at least 20%, probably not more than 40%). In that case, the risks of poisoning outweigh the possible stress of large water changes.