I think you are probably right about the tank versus swimming space issue a lot of us are faced with. Depending on how much rock you have and how you have it arranged, you might be able to keep a Kole tang; who are probably some of the best algae grazers of their family. To be honest, I think stocking an aquarium based upon a problem...which we all do...is 'iffy' at best but maybe you can check out some other options that would be happier without as much open swimming space. I can't say that I have any real good suggestions but I recently ran into some algae problems and after trying a couple different fish and even some lettuce nudibranches, I think I got on top of it by adding some red legged hermits and constantly picking off any bigger chunks of the hair algae that they wouldn't touch. So far they have been keeping it from growing back too much.

I am sorry to hear about your luck with these fish though. I think a lot of us have come to a point where we have to take a step back from wanting to keep a specific fish or invert so IMHO, you are probably doing the right thing by focusing on what you have and looking at other things for the short term future at least.