Rainbow Sheen

I am seeing a rainbow sheen on the surface of the water in a 40 gallon tank I am cycling. I am adding ammonia regularly for the cycle. Is the sheen the ammonia, some kinf of algae, or did something toxic get into the water? The cycle in progressing pretty regularly, so whatever it is isn't affecting the bacteria. I intend to completely empty and refill the tank and scrub it before adding the occupant (an RES), and I will test the waters with some feeders before adding him as well. Should I do anything else? It is running on a Fluval 3+ filter (it has maybe 25 gallons of water in it at the moment, I will fill more as he grows since he is small now).

Ammonia was clean ammonia from Walmart. I used it to cycle a 10 gallon and had no problems. The "sheen" is only on parts of the water, and it isn't really shiney or anything. It is dull, and I have to shine a light on the water to even really see it.

I use Prime, and I probably overdosed a bit on it. I lifted off the sheen with paper towels as suggested last night and removed the plastic shelf that seemed most likely (I hit it with a pretty heavy duty cleanser to remove some gunk on it, but I scrubbed like mad afterwards). This morning the sheen is back, and it seems to be centered over the container of lava rock I have sitting in the tank. I used it in my attempts at a homemade filter before switching to the fluval. Anyone ever had a problem with lava rock before? Could the dust be refracting light weirdly (I cleaned it as well as I could, but its just too dirty to get it all off)?