I just thought of one other thing, i know its odd, but it happened to my friend, anyway, what decorations did you put in, because for cichlid tanks they like hiding places, so many people stack rocks, but if you pull them from your yard and don't thougholy clean them there can be harmful elements on them that you wouldn't check for, but the other thing is that some people clean rocks with bleach and don't thoughouly rinse it all off, so those two other thing might be it.

The breathing hard is a sign of stress and can be from many things, it still kind of sounds like lack of oxygen, but i don't think they would show signs that quickly of oxygen deprivation. How hot is your tank too, because at higher temperatures the amount of disolved oxygen decreases

Yes i will agree with your lfs that a wet/dry filter does add alot of oxygen, so it probably isn;t that

Yeah, pretty much for right now, but i don't want to say something like take out the salt, oh they still died, well try this, oh they died well try this, you know. This is very weird, because you have oxygen from the wet/dry, the toxic levels are good, and even if (not saying you did it wrong) but even if the tank wasn't done cycling they shouldn't die that quickly, you said they were healthy from the fish store, maybe try something like doing a 30-50% water change and try cheaper fish, then when they are good go for cichlids. you said the water is 81 degrees, according to the heater or a thermometer, because in my tank i have the actual heaters set to 76 but the water is actually 82, i know im saying basic and general statements but i don't know what else to say. I will keep thinking because i am very interested myself and would love to see this issue resolved. I stay posted when i think of something else.

Wow, well i am glad the problem was found, sad that all your work was in vain, and hopeful you try again and get into this hobby. Good luck