"Slightly overstocked" is an understatement.
I'll get to that momentarily.
Clown loaches are notorious for carrying ich, and when they get it, it is bad. Being "naked" (= scaleless), loaches are highly sensitive to all medications. Salt should never be used with loaches. This is just a heads up in case the ich worsens, which unfortunately it might well do with the loaches. At this stage just keep a close eye on things as I mentioned previously.
On the cause of the cloudiness I cannot be certain. It may well be a bacterial bloom. I used to get a slight cloudiness in one of my tanks, only the one, and it would last for weeks then suddenly disappear for months. I never found the reason. Assuming you are not overfeeding (only once a day, and missing a day or two will do no harm). The plants will help in this, though those mentioned are slow growers which means assimilating less nutrients than fast growers would, but still a help.
OK, to the fish assortment. There are clear signs of stress here, and as I noted earlier, this is the major cause of ich and a lot of other heath issues too, since stress weakens the immune system. Has the Red Tailed Shark
shown any aggression toward the loaches? This could be as small a thing as just darting at them even if no contact is made. This shark species is best in a tank with no other bottom fish, especially those with stripes which seem to set them off even worse than normal. Have a read of our profile for more info, click on the shaded name.
The loaches are probably under stress from just having the shark in the tank. Fish send out signals, chemically, called pheromones, that other fish can pick up on. They sense the aggressor even if no physical action occurs. Common Pleco
attain 12-18 inches, and produce a lot of waste, both solid and chemical (ammonia). This could be a contributor to the cloudiness.
The eclipse catfish is not in our profiles. It is very aggressive, attains 18-22 inches, and is a real predator. It will eat anything that it can fit in its mouth. It may be small now, but here again it is sending out signals and the other fish are getting them--more stress. On its own it needs at least a 125g tank.
The group of clowns is good, but they need more space and soon, and away from the shark and eclipse. I really would consider removing the shark and eclipse soon, they are all but guaranteed to be real trouble.