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In the last few days I've found a few of my fish in the 55 gallon tank d.ead. They were 3 Angels (body size about a 1/2 $ size or slightly larger) and a Serpae Tetra.
The only fish that I have noticed having aggressive tendencies towards the Angels and Serpae in the tank were the 2 Yoyo loaches (about 1 inch to 1 1/4 " long) and a 1 1/2" Rainbow shark (Black body and all fins are red). The other fish that I have in the tank besides the remaining Angels and Serpae are clown loaches, small cory catfish, and a 2 Synodontis catfish. One of the Synos is a Eupterrus, the other is either the same or a Multipunctas. Since the Yoyo and the sdhark are still small, I doubt if they are the villans. Although I don't see the Synos bothering thwe other fish , I know they are nocturnal. COULD IT BE THE SYNOS ???
I have tested the water and found excellent readings for the Amonia and Nitrite and Nitrate. I have not added any new fish within the past month or so.

I would have gone for the shark, but as you say, it's a bit small. I wouldn't think it's the Syno's tho'. I have 6-7" long Synodontis decora with small angel-fish, and no problems.

It may help if you post the actual numbers of your ammonia, nitrite and nitrate measurement - we all have slightly different thoughts as to what is "perfect" or "acceptable", but numbers are in themselves objective.

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Mats

Mats, thanks for your reply.

The actual numbers for the amonia and nitrite were 0 ppm. Since I did not have a nitrate test kit I went to a lfs and had them test all 3 and the numbers came up the same. Sorry, but I don't have the actual number for the nitrate.

In the meantime, since I knew the culprit could not be the shark or the yoyos, I moved the Synos from the tank and put them in an African tank

yes i agree the red tail black shark (labio bicolor) can be a nasty fish i have had experience of one of these being the tank bully and had a go at most of the other fish except the cory,s !