I know 'giving deals' is almost a way of life for some dive shops and I honestly think this is one of the last places where people expect to haggle but I don't care for that. I am not turning down a discount if you give it to me but I would rather you look at your business as a business, decide what you can sell 'X' for and mark it for that price.
For me, when you start marking stuff down in front of me I have a few thoughts that run through my head. First, what is the real value of the item? If it was $85 but now you say you can afford to sell it for $65 then I think 'why not $45 or even $35?' and so I now feel you were trying to rip me off then and might still be trying to do it. Second, I then wonder what I could get for $85. Could I get that $115 mask for that? I don't know because the price is so fluid. I hate to haggle, give me your best up front, tell me why it cost that much (ie-quality vs other items in the store)
If you do want to discount, toss in a freebie that I need/want instead. Free air fills are always nice with the purchase of a tank.
Keep in mind, that is just me. Other people might like that surprise discount. It is very common but I don't think it is a good way to run a business.