While you can certainly treat the whole tank with clout, there are some issues with it. First is that it will stain parts of your tank blue. Namely the silicone. The silicone and tubing of the sponge filter are stained blue in my qt tank. There is no decor, so who knows what will happen to your rocks, heaters, filter intake/outputs etc. Also it will cost alot more, since on a 10 gallon qt tank a full treatment would be one tablet. But for 175 gallons you can see it would be much more. Another thing is that I've that clout can kill your biological bacteria in your established tank, but I can't confirm if that's true. But since clout is strong stuff meant to kill parasites, I can see how it would kill the bio bacteria aswell. Aslong as you can accept these things then you can treat the whole tank. If you have carbon in your filter you will need to remove it.
If all the fish are eating, then maybe you can try just the metro soaked pellets to halt any further progression and hopefully kill any parasites your fish may have. If some are not eating then you really have no choice but to treat the whole tank or go the qt route. If you have more than one affected fish I don't see why you couldn't just pull non eating ones and qt more than one at a time and treat that way, but that's not something I've ever tried myself. Good luck, i know it can be very hard losing your fish like this. I've been there, but since i found out about this treatment I have not lost a single fish to disease. Hope it works out for you!
