I have a large marine pure block in my sump, it sits under my chaeto so I never clean/touch it; been in there since this tank was set up about 24-27 months ago.
I have done ICP tests on my tank a couple of times, I do see Aluminum in the report, but not at any level that has led to alerts. Shows as not much higher than normal.
I recently through a Brightwell block down there, it is definitely harder than the marine pure block, other than that which is better I do not know.
I have spheres in my nem tank; they have been doing well in there as well. At the very minimum, they house a ton of bacteria and pods.