Yo Bro, your parameters look real fine, except for PO4. Try to reduce it to undetectable.
Looking at the monti, most likely is the lack of flow at the inner side, hence the portion inside receding over time. Montis don't need high flow, but make sure it's not at some dead spot. Look at the shape it's growing, the inner side will receive the least flow. Maybe turn it such that flow can actually flow into the inner side.
For now, my advice is to frag out the healthy parts, else the receding part may spread.
Will try to drop by ur place this weekend if i can yeh?
See ya!