No such thing as a standard >:(

ok so now that I have two fish tanks, and two sets of 2-foot lights, I thought well here's my chance to put the brand new tube into the deeper tank, as the newer tube is quite a bit brighter than the old one and would do the deeper tank much more good than the new shallow one.

So I pull off both hoods with the lights, wait for them to cool down, clean the covers off accumulated algae and pull out the two tubes that I want to swap. I put the tube from the deep tank into the hood of the shallow tank - and it doesn't fit!!! The two tubes - both exactly the same brand (Degenbao) and both supplied by the same manufacturer (Aqua One) - are 8 mm different in length! There is no way the tubes were going to be able to be swapped. So I put everything back the way it was, wondering all the while what's going to happen when I need to replace tubes ?? arghh!!