You can keep aquatic frogs with community fish, as long as there isn't a great size difference. I'm speaking of frogs, such as African dwarf frogs, for example, that live all their lives in the water and never come out on land. And, you need to be sure the fish are peaceful community fish, such as guppies, platies, tetras, etc. Amphibious frogs, which live both in the water and on land, should not be kept with fish.

If you get a frog, make sure it is an African DWARF frog and NOT an African CLAWED frog. The clawed frogs get HUGE, and will eat anything they see, and will outgrow your tank. My friend had one that was HUGE, and lived alone in a 55 gallon tank! The dwarfs stay small.

Here are some links for you:

http://allsands.com/Pets/SmallAnimal...cfr_rfb_gn.htm
http://www.aquamaniacs.net/aquaticfrogs.html
http://allaboutfrogs.org/info/housing/aquatank.html

Turtles are a no-no with fish. You might be able to get away with putting a baby aquatic turtle with large community fish for a short time, but turtles grow fast. Then, their fishy friends become dinner. If you want a turtle, they need their own tank.