Housing Discrimination complaint regarding Service Dog

[Michelle Fox]

I am the owner of a single family home in Lake County Ohio. I received a certified letter saying the Fair Housing Center conducted testing of my property. The tester informed me (owner)  that they had a service animal. In response to this they are claiming that I said a pet deposit equal to the monthly rent amount be provided. They are claiming discriminatory terms and conditions based on liability. Additionally, they are claiming they were frustrated and resources diverted due to my response. 
I do not recall having this conversation and am well aware that you cannot charge a pet deposit. I own several rental properties and have been in property management for 30 years.  I live in Ohio and am not sure how I should respond to this complaint. 
Has anyone out there had this situation?  Any suggestions on how to go forward? 
thank you!