I concur there are some serious stock issues, but I am going to ignore all that as you have a more urgent problem, ich, and I should be able to help with that.
I would use heat and salt. Clown loaches for some reason are especially subject to ich and once they get it often do not recover. But if this has been caught early enough, they should. Salt is far easier on these fish than any of the so-called ich remedies containing copper or malachite green or similar. I have treated cories, loaches and characins with salt (and heat) with no losses and complete recovery. I turned to this method after personal advice from no less an authority than Neale Monks.
This is a large tank (750 liters/200 gallons) which will mean more salt and increased effort to raise the temperature. So first, can you siphon off some of the water to a lower level and still have the filter and heaters function? This makes life much easier; I did this in my 90g.
Raise the temperature to 30C/86F. This can be done partly with raising the temperature of the replacement water of a partial water change (a WC is useful to start the treatment anyway) plus turning up the heater(s). I usually raise the temp half way via the replacement water and leave the heaters to do the rest. Having a very warm room helps, as it saves the heaters a bit. In a cool room heaters have to work longer to increase the water temp and this can cause them to wear out faster. Do a major partial water change, at least 50-60% but more if you can continue with a half or 2/3 full tank.
Add aquarium salt (not table salt, not marine salt, not rift lake cichlid salts, just regular aquarium salt) at 2 grams per liter; one level teaspoon is approximately 6 grams so that treats 3 liters. Dissolve the salt completely in warm water before adding it; never dump salt crystals in, though a few partially dissolved are OK. I add a couple teaspoons of salt to a jar of warm water and brisly shake until dissolved, then pour in and continue. Add the salt gradually, over several hours (within a day) up to what you need. Remember to calculate for displacement of water by substrate, wood, rock, etc.
One week is sometimes suggested, but I would maintain this treatment for two weeks. Do a normal partial water change after the first week, adding sufficient salt for the replacement water only. At the end of two full weeks, turn down the heaters and let the water cool naturally. The salt will gradually be removed with normal water changes. You can also top up the tank, but avoid water cooler than the tank water when you do, no more than a degree or two. Sudden chilling can re-start ich.
Feel free to ask any questions on the above process. This should not be problematic for the fish mentioned; the neons and maybe the cories (depending upon species) willnot appreciate the heat, but they should manage. Make sure there is very good (increased) surface disturbance; warmer water holds less oxygen, plus the heat and salt both increase the metabolism of all fish so oxygen is even more in demand. If you see any obvious problems, do a partial water change to dilute the salt. This should be OK for the fish if you don't overdose.
You should consider how this ich outbreak happened. Ich is directly caused by stress. Many knowledgeable biologists believe it is always present, and the fish can usually deal with it unless something causes them severe or cronic stress. It is important to identify this cause, so it is not repeated after the cure; also, if the cause is still present, it may not even be possible to cure the fish. Heat and salt will obviously cause stress, but this must be counter-balanced by removing all other stress causes.
Byron.