is the asp large core 18x12x3.5 or do those dimensions include the endtanks?
and if it doesnt include the endtanks, what how long is it from endtank to endtank?

 

I am pretty sure that does not include the end tanks, because the Medium is about 12"x12" (well, 11.5"x12") without the end tanks. The end tanks add about 3" to each side. So, 18 + 3 + 3 = 24". These are just rough numbers, though -- if you need precise measurements, it would be best to actually measure one.

-Max