It has been proven that heat build up drops the PAR...
I don't know if it is half like DarG wrote...
If you combine an acrylic shield and a fan, air is pulled in one side the fixture and pushed out the other creating a "tunnel cooling effect". this is obviously more efficient than a fan cooled fixture with no acrylic lens.. the fan would just be moving the air in the immediate area of the fan....
So it makes sense that even though you may loose a miniscule amount of PAR from the acrylic lens, having a acrylic lens on a fan cooled system is important.
if you have a fixture with a acrylic lens and no fan... you are not getting anywhere near the optimum lighting from your fixture... for 1, you are trapping the heat within the fixture (even if you have a couple slots..), AND you have the PAR loss from the acrylic lens...
if you have a system with no fan and no acrylic shield, the heat build up will be much less, but still enough to diminish the performance of the bulbs.. AND you get salt creep on your reflectors and everything....
In my opinion... The aquactinics fixture is a better buy... Better reflectors, fan cooled, and acrylic splash guard. Plus, supposedly much better service.