I think you have plenty of biological filtration with your ehiems and ac110. I agree that the sand is the likely source of the odor. You should be able to tell as you scoop it out. What type of sand are you using? If you are using very fine play sand from Home Depot, it may be too dense and growing pockets of anaerobic bacteria causing the odor. Pool filter sand is the best to use. I would also completely drain the tank, wipe down the sides and rinse the filter media in the canisters. After replacing the sand and water, let it run for a day or so and then recheck your parameters. You should be good to go.