Hey Fishie123, I have thought of a few reasons as to why you're having the problems that you are.
First of all i'm sorry to hear about your losses, from the pictures you posted they were nice fish. I apologise if this post is a bit longer than the preffered reading length all, I also apologise if I come off blunt or offensive.
As Phoenix mentions, doesn't have as much surface area as a tank with those specimens should. Although it shouldn't be the cause of death, It is something that should be reconsidered in the future maybe. I'm sure you were aware of that though.
For me, feeding regime looks good. Relatively well balanced, perhaps if you think you are overfeeding them. Try underfeeding them for the next couple of days and see how the L260 goes. I feel it isn't beyond saving yet.
As for water quality, with the filtration you have running I don't think that would be a problem. However, if you're not changing a good amount of the water weekly then problems may start to occur. Perhaps if you're doing 20% per week, try increasing it to two nicely spaced out 20% water changes. I do 20-30% changes every Wendsday and Sunday. This works for me, and have never had any issues with any of my fish.
This may sound silly, but I feel that the temperature of your tank is indeed a factor in these deaths.
One thing that's associated with temperature and water changes, is the amount of dissolved oxygen present in your tank. I feel deficiencies in oxygen can quite easily lead to fish deaths with no visible symptoms. "Water is an oxygen poor environment and contains only five percent of the oxygen that the same volume of air does." Leaving a tank with a decent amount of Plecos, all with relatively high oxygen demands (Well, i'm unsure on that. But some of the species you mentioned hail from waters with high oxygen content). The warmer the water is the less oxygen it can hold and only performing small amounts of water changes might not be enough to keep the oxygen levels healthy. Personally, along with my water change regime , my Temperature is stable at 27c all the time, with an air stone running 24/7 and again, am yet to encounter any problems.
There are a few other things that will lower oxygen levels too. Such as "organic waste, decaying plant matter, and certain chemicals also steal oxygen from our fish. When aquarists think of organic waste, we usually think of the dangers of ammonia and nitrite poisoning. That fact is, that in addition to being potentially toxic, these compounds are also stealing the very oxygen your fishes need to breathe."
I think it's possible that this cause of death has factors that were present with the death of your L270 also. It was being kept with discus, of which require warmer temperatures. If no air stone was present, then it may of been subject to oxygen deficiency aswell.
This post contains extracts from the following Shanes World article : A catfish keepers guide to dissolved oxygen
The article also contains alot more information on the subject then the my post did. So if you feel more elaboration needs to be made regarding the idea, that's where you'll get all the information you'll need.
This is all the information/speculation i can provide without being aware of the exact quality of your water, exact feeding amounts, etc etc. This is just what I deem as a very possible scenario.
Hopefully like I said I didn't come off too blunt, and I hope this information will help ensure future health of your fish.
EDIT : Thought of an idea, and decided to change my original post.