Spots on some of leaf ends, a little crispy

My plants are doing pretty well, have like 5 or 6 going well for 2-3 weeks.

they are perfectly green except one or two leaves on 2 of em are browning and crispy

i left the light on for maybe an extra 12-24 hours, and i water with tap water left sitting for a day

The plants are in a plastic bin with 2 flourescent lights on top, they shouldn't produce much heat, but i'm sure leaving em on for 48 hours wasn't fun for the plants

could there be any other problem? do i really need that much ventilation? they aren't completely covered or anything..

ventilation is a very important factor when you want to harvest a great yield .....in smaller grow's that are being grown with florencents or cfl's its not really that big of a deal but if hps is involved then thats different.....as for ur leaves being crisp.....usually light burn or nute burn ....if its on a leaf or two dont sweat it ur fine but if it does get worse ..correct the problem also use 18/6 lighting............