So my tank has been set up for about 2 months and been cycled for about a month. I have been slowly stocking my tank for that past month. Usually 1 species a week. About 2 weeks ago I bought 3 Kuhli Loaches (LFS only had 3 and these were large healthy ones). Last week, I went back because they had gotten a new shipment in. I bought 5 (accidentally got 6). Acclimated them slowly, tested water before putting in. Readings were good (0,0,5-10). The next day (Saturday) I found one dead. He had been in there probably about 4-5 hours by the time I found him. Pulled him out and tested the water. Ammonia was nearly 2 ppm and nitrites were starting to show. Did a 30% water change. Next day (Sunday) I test again. This time ammonia is about .5 ppm but nitrites were around 1 ppm. No dead kuhlis. Did a 50% water change. Everything back to 0,0, <5 ppm. Monday I didn't test nor were there any dead kuhlis. I didn't search around very well (I have a large cave that you can only see everything in there by picking it up and that's where most of the kuhli's stay) to see if there were any dead ones. Yesterday (Tuesday) I come home from work and there is another dead one in the tank. I scoop him out. Do a water test Ammonia 0.25, Nitrites 2-3 ppm (WHA????) Nitrates 5-10 ppm. I begin to do the water change and pick up the cave and there are 3 more small loaches dead. They were stiff and looked like they had been in there for quite some time. I do another 40% water change last night. This morning nothing is dead. The 3 larger loaches plus one small one are alive. Ammonia still around .25. Nitrite about 1ppm. No other fish seem to be struggling. WHat do you think is going on?