I did a search for this topic first but the answers seem to be all over the place. I have read some people saying up to 6 and some saying no more than one. Josh's Frogs says you typically want 5 gallons per frog but that seems like too many and I certainly don't want to stress the little buggers out. The tank is oriented vertically and is very well planted. These would be my first thumbnails so i'm excited.