Hey Randy, I have read your ozone article several times. Just wanted to see what you thought about some parameters I've been experimenting with. I recently moved my 75 gal tank in preparation for a larger tank I'm having built and fish room renovations. I decided when moving the tank to go bare bottom. For 20 yrs I've held onto the sand beds and refugium but after seeing how nasty the sand was I decided while breaking the system down to remove the refugium too. It was a nasty job. We were pretty shocked at just how nasty everything really was. I did a 80% water change with the move. I made the refugium an isolated skimmer compartment. I added a recirculating biopellet reactor made by reef dynamics and the recommended amount of ecobak biopellets and running 50 gph through the reactor. I have the effluent from the reactor running directly to the skimmer suction. When I got everything back up and going the only water the skimmer compartment was getting was from the biopellet reactor running directly thru the skimmer first. 50 gph. My ORP on Apex steadily climbed up to 450 range. I was scratching my head a bit. Years ago I ran ozone to try and reach 450. I checked the probes. But the tank is spotless. The sump is spotless so I figure it's just the absence of nutrients and large water change etc. All test readings are undetectable. 0.0 on hanna for phosphates. After a week of biopellets running I added a separate feed pump to the skimmer compartment. I figured I needed more water going to the compartment than 50 gph for my skimmer to process. When I turned the feed pump on (150gph) I noticed my ORP started slowly dropping in the tank. It dropped below 400. I turned the feed pump off and ORP stopped it's decline and slowly climbs back up to 450 range. So I'm assuming this is from the skimmer not processing all of the bacteria from the biopellets and more bacteria is being introduced to the tank when I upped the flow to the compartment. If this is correct, and I know ORP is very complicated, but could it be used to anticipate bacterial overpopulation. I know there is the BART-HAB test for checking bacterial populations. FYI the tank is very lightly stocked and I have an oversized skimmer. With the exact same equipment and sand bed/refugium my orp ran much lower. 250-300 range. It has been very interesting. The isolated compartment makes it pretty easy to see how effective the skimmer is working with different flow rates running to it. I'm just curious if the orp decline is bacteria being introduced into the water column or the skimmer not removing organics as efficiently with more flow to the compartment.