As pH drops under 6.5, biofilter bacteria may start shutting down or dying. That may result in a sudden increase in ammonia production. If there wasn't sufficient plant growth to keep up with it, this could be responsible for the death of your oto's.
I suggest increasing your kH with "Barrs GH Booster" from the same website where you got your dry ferts.
Or you can do it old-school with baking soda and epsom salts.