Sorry to hear about your Oto problems. My tank has been running for a couple of months now. I introduced 4 Otos after a month or so. Saw them all the time, until I did some more planting, which they obviously really didn't like. I then added two more. I often see two out and about, but have never seen all 6 together. I think they come out and feed at night, as cucumber, celery, courgette, spinach etc are often well eaten by the following morning.
Hopefully yours will stabilise. As mentioned above, snails are superb. I now have a very healthy breeding population of Ramshorns and they do a fantastic job, along with the Red Cherries.