I tried the same thing, but the cave had holes on the side and was utterly useless. But that is sage advice.
I've got two 15 watt incandescent bulbs for the 10 gallon and no algae at all. There are certainly enough nutrients for the algae to feed on (b/c of overfeeding and the algae discs) but there is no growth. I read somewhere that i should grow algae on rocks and put them in.
This failed b/c:
The rocks were too slippery and the algae came off b/f I moved it
There is no light here b/c it's really overcast (I could try a light bulb...)
I don't have rocks that are suitable for growing algae, and I don't want to place rocks in there from outside.
Any help?