Water volume is not so much the issue as swimming area. I know exactly where you are coming from, but after a few years in this hobby I have learned to trust the least favorable answers I get. LFS are in it to make money, so if they are telling you no then you should probably listen. It just means he has a bit more of a conscience than the other guy. LOL! I've successfully kept a lot of fish and passed them on in this hobby as I have upgraded tank sizes and ability to care for new harder to keep fish. I've killed a few, and maybe someday I'll be able to give something back to the hobby by breeding my b&w clowns. But my point is that this is one hobby that saying no doesn't mean never. Eventually most of us get a bigger tank, and with it the fish we always wanted. So, my advice... enjoy the space you have and do the best you can with it. When you one day get a larger tank and with it the years of experience to keep a majestic angel alive and happy, it will be all the more rewarding.