CO2/O2- you can kill a plant through suffocation, and the symptoms are darkened, shrivelled, thin-looking leaves.
Cold- A frost will make your plant look like that too
Cat piss- ehhh nitrogen overdose anyone? Symptoms are dark green lush growth, soft new growth, susceptibility to other problems (such as a frost)
Overly wet soil- root suffocation
Foil- lose it; replace with white paint, white posterboard, mylar, etc.
Pot size- One plant per pot, and allow plants to get a bit rootbound before transplanting so they are not sitting in wet soil.
Light- Those plants have little foliage for their size. Consider a larger light or more CFL bulbs.
Proximity to a gas-fired appliance- Not too safe. Should anything happen, you have a potential ignition source (elecritcal spark) sitting right next to a gas line. Might want to consider moving that.