HELP!!

[BMahura]

So i decided to reaquascape my 50 gallon tank and everything is now going downhill. After reaquascaping my tank yeasterday morning my water peramaters arent doing so well. i did a 10% water change after aquascaping was done. Last night my ammonia was at 1.5ppm, my nitrites were at 0.3ppm, and my nitrates were at 0. Today my fish seemed to be doing fine with the exemption of a damsel dieing. I did another 10% water change today and added a bit of seachem prime. my parameters are now almost 0 ammonia, 0 nitrites, but i now have 15ppm. My yellow tang is having lots of problems, its breathing really heavy and is laying on the sandbed, all my other fish are breathing heavy as well. i put my yellow tang in a tub with a drip of antibacterial medicine which gave him a little energy but he is back to laying on the sand bed.
I need help and any advice would be great.
Thanks.
Brett