I would start by isolating the fish and raising the temperature to 83 degrees slowly, and increasing water changes to every other day. Ich and velvet are usually only found on fish that have been weakened in some way. Just because one fish in a tank has it does not necessarily mean that all fish need to be treated--many believe that ich and velvet are found in almost all aquariums, but that healthy fish don't get infested because they have healthy immune systems.
Shining a flashlight on the lightbodied individuals should help you diagnose any velvet, look for crustiness in between the scales especially. Keep in mind that many lightbodied individuals are naturally yellow-green iridescent, don't confuse iridescent coloration with velvet, you should see crusty yellow bits between the scales all over the body if they have velvet.
Velvet thrives in a high light environment, cover the tank to deter the growth of more velvet. If darkness, clean water, and elevated temperatures continue to just make the ich and velvet worse, I suggest using a malachite green treatment, such as Jungle's Ich Clear. This works on both ich and velvet.
Keep in mind that there is probably a reason behind why these fish are sick--you should review your tank conditions and try to hunt down possible sources of stress that caused the fish to weaken.
As for sterilizing the tank, simply washing it out in hot water should be enough, vinegar would also help. For plant dips, I use a potassium permanganate solution. The easy thing about PP is that the depth of the color helps you determine the strength of the solution--I dip for about half an hour in dark pink (purple is too strong, light pink is too weak) potassium permanganate. You can find PP powder in the pool supplies section of Ace Hardware. This stuff is dangerous, and isn't always readily available, so I would call around before going to stores. Apparently glycerin + potassium permanganate = explosion. Fun.
Last edited by Adastra; 07-04-2010 at 02:23 PM.