contagious internal parasite?

[mcliffy2]

About 2 months ago my sargassum trigger, who had been healthy for 6 months, suddenly started behaving weirdly. he would come out at feeding time, but wouldn't actually eat. He almost acted as if he was blind and would only eat something if it literally hit him in the mouth. No external signs of anything except that he got skinnier. Two days later he passed. I did not add anything else to the tank, but a few weeks later the exact same thing happened and my sailfin tang passed away. Now it is about a month after the sailfin, and the same thing seems to be happening to my black saddleback clown. No outside visible signs, she comes out as if she wants to eat but doesn't. No other fish are affected. Whatever it is seems to be picking off fish one by one, which I would guess is characteristic of some type of internal parasite? What can I do? I am in the process of upgrading tanks and was going to acquire a QT tank this weekend, but I fear that might be too late. I have a cracked nano cube that I could possibly silicon up tonight so I could try to catch the clownfish (hoping there is a chance I can catch the guy since he seems a bit lethargic due to lack of food). So obviously, the first thing is to try and get her out of the main tank, but then what, not sure what meds I could use? Do I do hypo?

 

[mcliffy2]

I think I figured out the disease, but now I don't really get how I'm supposed to administer the treatment. This sounds like exactly what I'm experiencing (while there are some external symptoms, it seems like they might be hard to spot):

Digenean Trematodes
These internal worm infections are actually quite common in the wild. Most fish appear to be oblivious to the presence of these parasites unless they are in an otherwise weakened condition. It is surprising how many of these parasites a fish can host and still appear to be healthy. It has been reported that Trematodes, Metacercaria, or Nematodes are found in up to 85 percent of marine fish examined. At the risk of spoiling someone's appetite for fish, try soaking a freshwater fish fillet in saltwater or the saltwater fish fillet you are planning on as dinner in freshwater. You may discover that some types of worm infestations are more common than you would like to know about.

Some possible symptoms are as follow:

• Weight loss, even though the fish eats normally in early stages
• Usually only one or two fish seem to be affected
• Loss of appetite in advanced stages
• Fish become listless
• White spots that are most easily seen in clear fins appear to be imbedded in the fin rays
• Dark spots under the skin that do not respond to treatment with Formaldehyde
• Exophthalmos (popeye), or blindness
• Secondary bacterial infections in late stages

And here is the treatment:
For Digenean trematode infections Piperazine is a good choice, which should be administered in food at 4.5 mg per pound of body weight each day for three days (Post, 1983). This can be a little difficult to measure for most aquarists and treatment is not always warranted; therefore, improving environmental conditions should be the first approach.

My water quality is very good. All params are good, and I do religious weekly water changes (my corals have been growing like mad lately). But how on earth am I supposed to figure out how much my fish weigh??? And if you can determine body weight how do you administer such an exact amount by food? If you do it by food, can you just feed in the regular display....ugh, this is not pretty...