I absolutely love L-260!! What tank do you have them in and what filtration, tankmates, feeding ETC as I am considering getting a group for 1 of my tanks but really want to make sure I do it right. Notice you have them in with other plecs, is that going to be long-term or not???

At the moment I have 3 L260s with some L333 juves, 3 tiny Licorice Gourami and 5 Red Tiger Badis in a 40 litre tank. Seems like a lot of fish but they are all very small. They all have the same basic needs, low pH, high temp (apart from the Badis which need low temps and they will be moved to my new sub-tropical nano tank), live foods... The plecos will be there short term as they are too small for my 125L & 200L tanks where they would be killed by now. Filtration is 350LPH so just under 10X turnover. These plecos love high currents
Have you tested the water to see if anything else could be the culprit, going by the above that is alot of fish in such a small tank especially that many plecs no matter what the size are they produce alot of waste.
Plus they have grown, plus they are meat eaters so foods uneaten and left over food that havent been noticed will pollute water more and then adding a couple more fish like the betta's was it could have just pushed everything over the edge?.
The filtration has no relevance on the tank turnover if the media can't hold a colony of bacteria big enough to cope with the bio-load.
Also like simonas find it quite hard to believe that the addition of a couple of betta's would cause such damage, especially to plecs which are extremely quick and agile while young and expert hiders.
The reason you may not be losing more fish is due to the tank losing stock, the bacteria may be sufficient for the stock currently in there and no more. with the other fish, it may have just pushed it over the edge causing slight fluctuations in parameters and young plecs can be very sensitive to this.
Me wouldn't have had so many fish in a tank never mind so many plecs at the same time. I'm sorry to say but if was the only way of growing these fish out (cramped conditions) then i just wouldn't have brought the fish in the first place

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Not meaning to sound harsh but it does sound like you may have caused this issue yourself and the poor fish suffered because of it