Considering that the smaller fish appear to be fine, I would think they are getting proper amount of food where perhaps the larger fish, may not be. The fish you have chosen are carnivores in nature and require meaty foods and lots of it as they grow larger. foods like carnivore pellets, earth worms, beef heart, and you might consider raising some feeder fish in another tank as opposed to buying them and take the chance of introducing disease in that way.These fish , Like most,, Do best in well maintained water conditions. Even small spikes in ammonia and or nitrites will harm fish and the ones you have chosen are quite sensitive in that respect. Were it me,(and it ain't) I would not replace the columbian sharks which require slightly brackish water as they mature with fish that might do better in clean freshwater containing no salt. I WOULD provide all of the mechanical and biological filtration I could to this tank in the form of additional filters whetherthey be hang on the back such as Emperor 400"S or the addition of canister filter ,the larger the better. I would also add aeration if you aren't already,, in the form of air stones and or spray bars. As mentioned in a previous post ,I would also employ the use of power head as well for movement of water. And lastly, and most importantly,, I would change the water at intervals and at such volumes,, that ammonia =0 nitrites=0 and nitrates read less than 40 with 20 being even better, all day,, every day. I do hope your fish improve and hopefully, some of what I suggest will prove helpful.Most of these large catfish are slowly suffocated due to lack of proper filtration , lack of proper foods,and or lack of clean water.