If you're reasonably certain that the turbidity can be attributed to suspended organics, then the TDS pen is applicable. My recommendation for the turbidity meter is that it should be used in setups containing inorganics. Materials such as clay and PCC would not read on a TDS pen.You would be able to test the conductivity of tap vs tank water and get an idea of how much more organic matter would be in the water though. I'm thinking that for a practical approach, if turbidity would be able to be assessed and controlled through water changes with a $10 tds pen. I assume state water has regulations about turbidity but I'm not sure in what requirements have to be met. I just wasn't sure if the conductivity would be a realistic approach. The idea has been thrown around to monitor when you should water change to keep acceptable nitrates and stuff, but I'm confident that stuff shows up on the pen.