The 1 I can see clearly looks to be a juvenile botia kubotai (angelicus). At 4 or 5 inches , a good size for your tank.
Loaches are not the best for cycling a new tank...Did you use "used" filter media"? Or at least "safe start"? Live plants?
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Stock nothing until you know what your water parameters are. In other words, wait for your test kit. As you've seen with your comet, poor water conditions are lethal.
???My comet is 7 years old and doing fine. He lived in a 1gal bowl for 2 years, then in a ~6 gal bowl for 5 years, only had a filter for the past 6mo (didnt know they needed one), now hes living in the tank til I get some new fish.
Ive got a few tetra test strips, doesnt test ammonia, Nitrate is between 0 and 20, nitrite is at 0, GH at 150, chlorine zero, KH 80, pH I know is 6.95
The way you indicated the 15-20 days, it implied, to me, that the comet had died.
Strips are known for being less accurate. Get the liquid kit, test the water, and proceed from there.