I am new to Hydro, and I had the same question when starting. I wans't sure what the cutoff was when it comes to a need for an RO system. From my research and asking experienced folks, they recommend that you purchase an RO system (I think the smaller ones go for abround $250-$300) when your PPM of tap ater meets or exceeds 200 PPM. You are near double that, I would go with RO to be safe. My hydro growing is just getting started because I wanted to wait until I had the funds to do it right, I figure with th money and time invested, might as wel do it right or not do it at all...but I am in the learning stage, so it is best to consul with more knowledgabl folks than myself.