The same brand stand is required for warranty.

Proof of purchase... that's a little fuzzy. Who keeps a receipt for 20 years? An oceanic tank looks like no other... a sales rep will know its an oceanic without your receipt saying so. If you can get them to cover your warranty depends on your relationship with them. The old oceanic tanks were quality, but the new customer service isn't what it used to be. It can't hurt to send an email and see what they say.