When I got back into the hobby I had the same issues with Oto cats. How long has your tank been running? They need a well established tank. Personally I would also recommend at least 20-25% water change per week. Do you treat the water when you do a water change. They are also quite sensitive to chlorine and other stuff found in tap water.
What I also found out is in general if not always they are wild caught and the shipping to your lfs
takes a toll on them. The secret is to ask how long they have been in the store. The rule of thumb is minimum of a week to 10 days. Also you need to look and make sure they look healthy their bellies should look slightly rounded not indented.
A trick I was told if you take 30 min to acclimatize a new fish to your tank an oto cat needs 45-60min
I now have 4-6 in my 70 gallon they have been happy and healthy for years. You need to expect to lose one or two at the beginning.
Hope this helped
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