How are you measuring p04? Since nitrate is removed much faster than phosphate I think it's strange that your po4 is 0. In my experience the meantime pure block kept my nitrates too low, and I think It caused a nutrient imbalance with higher po4. I threw mine out and my tanks been doing better ever since. If you put the block in ro you killed all the benificial bacteria. I would throw the thing out or at least remove it and re cycle it in another container. All the die off in there is gonna take awhile to get rid of. Those blocks are so porous it's gonna be hard to get that clock clean. If your going to cycle it I would use lots of bottled bacteria Gfo and water changes in the container used to cycle. I think it goes without saying but I would use a heater and a pump a skimmer in there would be awesome too. Good luck.