Diverchik:
that's what I'm asking. Do you change o-rings between every fill?
No, you don't change it every time. This would ruin the tank valve in no time, especially if the o-ring change is done carelessly. I've seen a lot of people jam an o-ring pick made from steel into the gap between the o-ring and the seat. This scratches the seat and ruins it over time. The correct way is to 'spear'
the o-ring without touching the metal and pulling it out of the groove. Ideally, one should use an o-ring pick made from brass, which is softer than the steel the tank valve is made of.
Before you put your reg on the tank, do a quick visual check of the o-ring, Wipe away any dirt or debris and check for a smooth surface. If the o-ring looks 'ragged' anywhere along its edge, exchange it. That's really all there is to it.
As far as leaks go: If there is a slight 'champagne'-like fizzle consisting of tiny bubbles, don't worry about it. It is safe to dive and pretty much every other tank I've rented did that. If you want to be really good about it, change the ring after the dive. However, if the bubbles are larger, pinhead-sized and up, I would exchange the o-ring before the dive, although people have told me not to worry about it even in those cases.
You can try the following to check for leaks before a dive: After you have visually inspected the ring, put your reg on and turned the air on, and spit into the groove between tank valve and first stage. If there are no bubbles, you're fine. If there are bubbles, turn off air, bleed the reg and take it off, then wipe the o-ring and put the reg back on. Sometimes this is all that's needed to stop a slight leak.
Hope this helps.
