1.) That depends on how many frogs you have but since you should have at least a pair I'd recommend at least a 2-gallon. In a 5-gallon you could have five frogs.
2.) If you have a tank smaller then 5 gallons then it can't really be cycled so a filter doesn't serve much of a purpose...and with frogs it can be a bad thing. They are very sensitive to strong currents and it can shorten their lifespan if they constantly have to swim against the filter current. It would be like if you were constantly in the middle of an earthquake. They aren't very messy either and come up to the surface to breath so that isn't a problem. Just remember to change out all of the water at least once every few weeks and rinse the substrate. On the weeks that you don't do a 100% water change it would be a good idea to change part of the water and remove uneaten food. Also vacuum the gravel if you have a gravel vac.
3.)
African Dwarf Frogs often don't eat when you're paying attention, hehe, and it can be hard to tell. Obvious signs that the frog isn't eating are seeing uneaten pieces of food and the frog losing weight. Some don't like certain brands of pellets so you might have to experiment with different kinds of foods. Make sure the pellets sink since they won't come up to the surface to feed.
4.) I would get another one if you have room since they seem to enjoy the company of their own species
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I hope that helps!
