If soluble aluminum is released, and it hits 0.5 ppm, soft corals close up. I didn't see that happen at 10x lower (0.05 ppm or 50 ppb), but I do not know the long term consequences at lower levels, and there is some thought by some people that the aluminum released from similar materials (like alumina) is actually very fine alumina, not individual aluminum ions.
This has more on those tests and alumina solubility (but Marine Pure BLocks are a different aluminum structure):
Chemistry And The Aquarium: Aluminum In The Reef Aquarium ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/2003/7/chemistry
from it:
Biological Testing Of Aluminum Exposure
In order to more definitively show whether aluminum released by Phosguard might be the cause of reaction in corals, it seems prudent to test aluminum on corals. Toward this end, I set up a 30-gallon aquarium with several corals. These included a leather coral (
Sarcophyton sp.), green star polyps, and brown mushroom corals. In addition, the tank contained sand, live rock, and some macroalgae (
Chaetomorpha sp. and
Caulerpa racemosa ).
Pictures were taken of the several corals, and then aluminum was added. Figure 3 shows the leather coral before any addition. The first aluminum addition boosted the aluminum concentration by 0.005 ppm by adding 0.15 mL of an aluminum chloride solution to the water. A pH electrode was in the water at the time and recorded no pH change (pH = 8.32). There was no apparent change in any corals in 2 hours.
The second aluminum addition boosted the aluminum concentration by 0.045 ppm by adding 1.35 mL of an aluminum chloride solution to the water (total additions = 0.05 ppm aluminum). A pH electrode was in the water at the time and recorded a pH drop of from 8.35 to 8.33-8.34 pH. There was no apparent change in any corals in 24 hours.
The third and last aluminum addition boosted the aluminum concentration by 0.45 ppm by adding 1.35 mL of an aluminum chloride solution to the water (total additions = 0.5 ppm aluminum). A pH electrode was in the water at the time and recorded a pH drop of from 8.35 to 8.25 pH. There was no apparent change in any corals in 1 hour. By the 5-hour mark, the leather coral had closed (pH = 8.30), and then it began cycling between open and closed every hour or two, continuing into the next day. Figure 4 shows the closed form.
Thirty six hours after the last aluminum addition, the leather was still cycling between open and closed. This behavior had not been exhibited by this coral prior to aluminum additions. While I cannot be certain it was a result of the treatment, it seems likely.
After 48 hours, the leather no longer opened at all. It then stayed closed for the next 3 days until the termination of the experiment. I’ve since moved it to my main tank in the hope that it will recover.
5 hours after the last aluminum addition, the mushroom corals appeared less expanded than before the aluminum additions, but not nearly as dramatically as the leather. They stayed that way until the termination of the experiment.
The green star polyps seemed unchanged for the first 48 hours. After that, they expanded significantly less than they had previously. The polyps were about half of the size that they were before dosing aluminum. They were still that way at the termination of the experiment.
