That's because you were snorkeling in the wrong place - it has nothing to do with the aquarium industry. Go to the right area, and you'll see piles of them.
Aquarium fishing on the Big Island is already prohibited at the popular & accessible snorkeling spots. The aquarium industry doesn't have an effect on the fish on those areas. If you didn't see any yellow tangs in the place you went, it's because the area wasn't a good place for those fish. Every fish (yellow tangs especially) needs a specific type of habitat to thrive. Not every snorkeling spot is the same, so it's natural that some places will have a lot of yellow tangs and some places will have none.
It is true that Bob does care about the reef (in his own mind). I have known people who rented gear from him and were told to avoid some places because they were too crowded. Just like us, it's not in his best interest to destroy the reef. However, I've also seen plenty of bumbling tourists crushing marine life while carrying bags emblazoned with "Snorkel Bob Cares". Morevoer, the biggest threat to the reef (on Maui where Bob lives) is from the island's horrible sewage system that leaks effluent on to the reef and is incredibly damaging - this has actually been well documented. The reason this is a problem is because the island's growth hasn't kept up with its infrastructure. Meanwhile, the people staying in the condos and hotels and flushing the toilet are the same ones that the snorkeling industry depend on for practically all their business.
I don't personally have a problem with people snorkeling, staying in hotels, or doing whatever else tourists do that affects the environment. Saying that we should ban tourism is just as ridiculous as saying we should ban fishing - there's room for everything in moderation and as humans we need to figure out how to balance our activities with nature. What I do have a problem with is the level of hypocrisy that accompanies the whole thing. After Snorkel Bob started his campaign several years ago, several of us tried talking with him personally to see if there was any possibility of compromise. His response has always been that he doesn't care how many fish we take, he's doing it because he thinks the fish are his friends and it's mean to take them out of their homes; as long as we're still allowed to take one fish he'll still go after us. That has nothing to do with taking care of the environment - it's a personal issue that's driving the whole thing and it's the reason why they're trying to ban the fishery instead of pushing for reasonable fishing regulations.
Why do you say this? I catch aquarium fish myself and the survival rate of my fish (until it reaches the fish store) is close to 100%. I know this because if my fish have problems, I'll hear about it later on and won't get paid for them. Last time I asked one of my customers how many of my potter's angels he'd lost, he said it was about 1 in 500. We take good care of our fish and don't have any significant mortalities. Sure there are idiots now and then who don't know what they're doing and end up killing a bunch of fish; they are few and far between and rarely last long since it's a poor business model. People like "jimmy frag" who have had bad experiences should vote with their wallets and make sure nobody buys fish from bad wholesalers - it'll do all of us a favor. The last time the state did a study on fish mortality the absolute high estimate from reef to the mainland was something like 4% - IMO that's higher than what should be acceptable but it's still a far cry from what Bob & his friends are claiming.
It is true that there are places in the world where fish husbandry is pretty bad and a lot of the fish don't survive to reach the hobbyist. Hawaii is not one of those places.