Welcome to TFK, tiger. I'm sorry you're having problems with your fish. Unfortunately you have too many fish, and a bad combo as far as species, but you already know that. Your cats and sharks WILL NOT tolerate the heat/salt as Romad states. Also, with the scaleless fish you have meds should only be used at 1/2 strength. Get them healthy and then get them moved to an appropriate sized tank.
What brand of medication are you using? When I treat for Ich I follow the medication instructions to the letter (except for using at half strength) and the meds I use don't require a water change until treatment is completed. However, since I use at half strength (due to loaches and cats) it takes longer than a week so I do a large water change weekly.
Is there anyone in the tank that you are planning on moving them to? If that tank is larger and uninhabited I would consider moving them to the larger tank and treating them there. There is the stress of a move but they are in considerable stress now being in such a small tank and with so many fish.
Here's an article on Ich that is my favorite, which I think anyone experiencing Ich should read: The Skeptical Aquarist
Tell me more about the larger tank. Is it cycled? What size is it?
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