Yes, I'd agree that it would be wise to limit supplementing with fish oil to those times when you aren't getting enough omegas. I don't know if you can get too much, but if you are currently eating enough fish and other sources of omega 3 and omega 6, then it's not necessary to take a supplement on a daily basis as well. But you should definitely find a good supplement for those times when you don't get enough. And speaking of the mercury issue, you also need to make sure your fish oil supplement is a good quality one with any mercury or contaminants removed. I found a nice one called NeuroDHA
that has removed all contaminants, and also has a great EPA/DHA ratio to make sure it's exactly what the body needs and is easily absorbed.