Planted Tank Guru
Join Date: Nov 2009
Location: Missouri united states
With pH posted, ammonia would be less of a concern than with pH above 7.0
Is not uncommon for otocinclus which are still mostly wild caught, to do poorly as result of capture,shipping,and then acclimation to dealer tanks, and then acclimation to your tank or mine.
If the plants are doing well,and water parameter's are as posted,you may not be doin nothin wrong just fragile fish,stress,led to the oto's demise.
Nerites are sorta iffy also.
Some sources indicate that the nerites do better in slightly alkaline water with maybe some marine salt added at weekly water change.(shrimp's might not appreciate it)
Opinion's vary on keeping these snail's long term.
I do believe the tank is too small for anything but the shrimps you mentioned,and that the small tank was designed around.
I would perform weekly 1/3 water change with dechlorinated water and maybe try some ghost shrimp and see how they do before committing to the CRS.
All bet's are off if using buffer's that might cause fluctuating water parameter's for nearly all critter's.
Just need to get the tank and you into routine of regular replacement of a portion of water while resisting the urge to over feed possibly.
Personally,,i would opt for a tank of at least ten gal which would afford more room for maybe some small micro fishes and help keep water quality more stable.
Hope some of this helps.