The Marinepure block explains the Aluminum. MP is known to leach Aluminum. Google "Marine Pure Aluminum". Here is my triton result:
At that time I had a 8x4x4 Marinepure block in the sump.
I would suggest running Poly-Bio Marine Poly-Filter. There are some other copycat "polyfilters" in the market. Make sure to get the Poly Filter from Marinedepot:
https://www.marinedepot.com/Poly_Fi...Media-Poly_Bio_Marine-PB1111-FIFMMEPM-vi.html . This thing has a very high binding capacity for copper, but it also binds many other heavy metals including Aluminum, Lead, Cadmium, Mercury, Zinc, Iron. It has been extensively laboratory tested for its efficiency. I usually like to run a Poly Filter once every 2 or 3 months to remove any potentially accumulated heavy metal or toxin that I dont know of. I run it for 2 weeks and then throw it away. Don't run it all the time because it can potentially strip the water of some trace elements such as iron. Even copper may be beneficial for corals at very low levels.