Your lighting is fine it's only 85w over 215litres.. that's actually low (<0.5w/litre), I prefer 1w/litre or more. I'd recommend some hardy slow-growing plants like java fern, you can even tie it to a rock, and it can survive algae attacks. you should at least that way get something growing to help to absorb excess nutrients
I have an idea, those sponge cartridges you replaced, do they have carbon inside them? i came across some that did, possibly they are leaching phosphate (as you mentioned earlier re carbon). try the PO4 tests again on a bucket of tap water and bucket of tap water with the cartridge submerged, test every second day for a week or two
I'd also recommend doing lmsmith's tests: