I only ever used prime except for my constant water changer on my 480g I ran through two carbon blocks. I never noticed any issues with any of the water. I did, however, feel like my most recent 210g FW tank had Ph issues. I may have had issues at my old house with the 2000g of water I had in my basement but the 210g in my living room didn't keep the fish happy. I am pretty sure it was a ph issue. Something that I think may have been the cause of death of many rays a few friends and I tried to keep. When I really started looking in to it it seemed like our very low ph, which is good for rays and most of the South American fish we were keeping, was also very low in alkalinity. I think normally that's an issue with ph drops, which it could have been, but I think it's more likely that it was spikes that they didn't like.