Descaling a top loading washing machine without a built in heater

You have a crack somewhere in the drainage system. The most likely reason that it's leaking on the spin cycle is the vibration caused by the spin cycle is opening the crack to let the leaking occur.

[It's very probably measured on high] edit - nope there's an average of sorts - if you'd like a very tedious read, the (US) government procedure is here, at least for some point in time. Beware, it's govenmentese...CFR-2011-title10-vol3-part430-subpartB-appJ1

It takes considerably more than two tubs of water to wash and rinse. Rinses may be multiple, and there may be water sprayed into the spin portion of the cycle for additional rinsing. In many cases the "clean" clothes will still make suds and or dark water if you run another wash cycle with no new detergent, so even that much water may not really rinse things "clean" other than the sense that we normally accept that when the machine is done, we call the clothes clean...

This is a good quick read about washing machine test standards, but does not really address "what size load do they test on" from what I can see.

My dishwasher actually told use per cycle options, and that made me suspect that there the "normal" option is the one in that standard, as every other cycle used more water.