Some of them (mediums+ usually) have a section in the lid that you can pop out and pass through cords or airlines. The lids seem to vary from store to store. The medium (1.5 gallon) I got from Petsmart has only a tiny hole in a corner for an airline. The same size one I got from Petco has a circular section (about 2 inches wide) that you can pop out.
However, I usually just leave the lid on, but not secured, with the cord for the heater sneaking out on one side.
I also have a small one that I have for transport or emergencies, it has no holes at all in the lid, so you'd have to DIY that one if you wanted a hole.
Oh - heaters. The medium+ can take my fancy settable heaters just fine, I put them on the back wall at a shallow angle so they fit. The small one can *technically* fit the nice heaters if you get creative, but I think I'll just use one of my small non-adjustables in it if the need arises.
Last edited by Shadyr; 07-19-2012 at 04:41 PM.
Reason: forgot some info