It can be the dust from the gravel, as in the case of your tank. Estes coated gravel is beautiful. It does chip and I imagine would be hard to boil and keep as beautiful as when it was new. (Solution, partially change water regularly and don't get any sick fish!) ;)

New tanks will also cloud from an bacterial bloom. Sunlight can increase it and you putting up reflective foil is a clever way to deal with that. The safest, but ugliest tank backing is white paper or posterboard. It will not get hot and convey that heat to the tank though.

Another possibility is to put the tank background of your choice on the tank. Behind it, put the white poster board.

I have seen some awfully sad tanks with "new tank syndrome." The more slowly one stocks a tank with fish and other animals and the more stuff brought in from seasoned tanks (gravel, water, filters...) the less danger of clouding. If the water clouds again, cut down on feeding and increase the gentle gravel vacuuming.

Read recently of a fishless seeding of freshwater tanks where a measured amount of ammonia or something purchased from a LFS is put in the tank,  to get the show on the road. I'm sure your shop keeper can tell you (and sell you) more. Others here may have some experience along that line. In that the tank would be safer for the small group of fishes after 2-3 three weeks of adding that stuff and measuring for ammonia and nitrates, that  sounds like a really humane way to break in an aquarium!

By the way  - and you knew this was coming :) - take a gander in the GL Immediate Help section at Water (cloudy/green etc.). There are more thoughts there.