Brown algae

I have mega-ultra-crazy-low light on my tank. The tank is 30 inches left to right, 18 inches front to back, and 24 inches tall, the canopy keeps the lights 5-1/2 inches off the water. I have two F17T8 bulbs currently running with no overdrive at all, which from Hoppy's charts doesn't even register at all in the lighting level discussion. All this, yet I've got a thin brown film on all my rocks, the shells on the bottom of the tank, the filter tubes, heater, everywhere.

It got really bad one day and I figured I needed to add a herbivore of some sort. I added 4 ABN pelcos (they were tiny, hence 4). They knocked it out within a day or two so I decided to give a friend of mine one of them because he didn't have any algae eating fish and was going to by a standard pleco for his 20 gallon (if only they put how big these fish get). One of 3 remaining died. The algae is now back, top to bottom in the tank, and the 2 remaining ABNs seem to have no desire to eat any of it. What do I do?