If your tank is surrounded by oil based vapor, then some of it will defintely enter the water as it settles, being heavier than air. One drop of oil can pollute 1,000,000 gallons of water, imagine how little it would take to ruin 100 gallons. I painted a room with an open top tank in it no problem with water based killz. Make sure you open a window and have a fan blowing out it.