After living in a rural area for 18 years, yesterday we experienced our first frog in a commode. After speaking with neighbors, I have learned this. They are coming in from the venting pipe over each bathroom. We are on septic not city waste management. They are tree frogs, that jump into the venting pipe and make their way to the water in the commode. Hire someone to cover all four venting pipes in the roof with fine mesh!
I googled your question and here is a link I thought might help answer your question. Good luck!
http://ths.gardenweb.com/forums/load/bath/msg071252026837.html?10
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