On dendroboard.com people are often advised to make water shallow as dart frogs apparently do drown in deeper water. They are terrestrial frogs that like wetland environments but are unable to swim. But, I do think it may have to do with the steep "high" sides of the water sections. If things are gently sloping, I do not see evolution making dart frogs pull the proverbial Lemmings trick (not real I know) and drowning themselves.
Something like this perhaps:
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As opposed to this one:
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This looks to meet in the middle-ish:
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This would allow many critters safe access to the water and easy-ish access for them out of it.