Very similar problem
I have been following this thread as I have had a very similar problem cycling my 75 gal tank. I followed my LFS advice on cycling it and think I may have payed the price for it. I knew they were wrong, but they insisted I do it their way for it to work. Anyway, here is the story. I put six damsels in for the cycling process. I used lace rock and play sand for the current decor and substrate. The only live rock availble to me is at minimum $50+ a rock. So back to the story. They said to put the damsels in and just let it go. Do not do any water changes until you show zero for your Ammonia and Nitrite. Then do your water change. I knew right away that was wrong, but I figured, they are the experts, I will do what they say. Well, four weeks later, I have no damsels (all dead by week two) and two I could not find after their death. I think this is where my biggest problem started. My ammonia was up to 5ppm and my nitrite and nitrate were extremely high as well. These levels remained the same for two weeks now. The ammonia would not drop whatsoever. I moved all my rock to see if I could find the fish that died, but never could find them. So I have no dead fish in my tank, as they must have decomposed completely already. I decided to try what was said in this thread. I hope I didn't do wrong again, but I tried a 50% water change. That seemed to have worked partially. I was still just below 5ppm. So I got really mad and decided I would do an almost 100% water change. Basically down to the sand. This was last night. Put the new water in and decided to check the ammonia level right away just to see what it said with the new water. I am finally down to 3ppm now. How many water changes do I need to do before I get this ammonia under control? I have brown algae all over my tank, rocks, and sand. After my water change last night, the algae on my glass turned green.
Please help me if you can. Oh, I almost forgot to add that I bought some of that live sand in a bag around week two to see if that would help bring the ammonia down. It wasn't much, but I figured I would try something. Thanks for your time.