I have used galvanized tanks for years – but stopped using them for growing a number of years ago as my gardening needs expanded. In my experience – do NOT put holes in the tanks unless you coat around the holes and the edge of the hole with a silicone rubber sealant. If you do, believe me, you will speed the rusting of the tank – the coating gets compromised, see? If you grow acid loving plants the coating will break down faster – but they will last you a very long time – so no worries really. It’s the holes in them in not coated will speed the degradation quickly.
If you live in an area that can get lots of rain, and/or days on end – then make the holes but coat them. Otherwise, a deep layer of gravel (I used 8″) with fine mesh or cheesier landscape fabric (very lightweight) to keep the soil from migrating easily will do.
The things to think about is1) how hot, when placed in full sun, the zinc will heat up the soil. The zinc tanks when new will blind you sometimes in the sun!! (Also draw attention BTW) – 2) how much soil it takes to fill them. A 100 gallon tank, for instance, will take 600 pounds of soil/material (or 15 – 40lb bags) to fill if I remember correctly, if you aren’t using a gravel layer. HTH
I use my old stock tanks (with holes plugged) to make my compost quicker. I add straw, leaves, horse manure, garden waste, cut up small twigs and branches (run through chipper) into one tank until it is half full, and fill the rest of the way with water and literally let it rot. Then use a pitchfork and shovel to transfer it into another empty tank to dry out, turning it once a week or so. Then I add earthworms – let it sit a couple of weeks more – then use it. Quick.