It is sold as "Bird Biotic" for $11 for 30 or $20 for 100 100mg caps at <a href="http://lambriarvet.com">Lambriar Animal Health Care</a> without a prescription for animals. 100 mg will treat about 4 gallons per day given the routine I used. So, for $20 you get enough to treat about 400 day-gallons from those guys.
I also found a place in Savannah, Georgia called <a href="http://www.globalpigeon.com/main.html">Global Pigeon Supplies</a> that sells a liquid preparation that is enough to treat 800 day-gallons for $22.95 + shipping. One ml of the liquid is 20% doxycycline by weight so I figure 200 mg. That means one ml of that liquid should be added to 8 gallons or so of water for the treatment bath (and, as above, the water has to be swapped out each day and re-treated to stay effective in medicating your clam).
In either case, I doubt the exact dosage is really that important. If you use twice the amount or half, it probably will work either way. But there are some guidelines that I'd follow:
1) Treat the clam in a separate bucket or tank. Antibiotics introduced in the main tank will kill off your biologically filter and crash your tank.
2) I treated my clam for 5 days. I would recommend you treat for AT LEAST 5 days. Even a week or more is not out of the question. Antibiotics do not work well on a "dip" basis. And the clam very well could look perfectly fine after a day or two. DON'T STOP THE TREATMENT PREMATURELY! If you stop the treatment fast, you greatly reduce your chances of the clam being cleared of the infection sufficiently to survive.
2) Treat the clam in darkness or very low light, and provide a phytoplankton food source each day. This is because the tetracycline family of antibiotics break down in bright light, and your clam will need some form of food to get well. <b>Use vinyl tubing and an air pump to keep bubbles rising and moving the water around (use a powerhead if the tank is large enough but not in a few gallons as the temperature can go too high). Use a small heater to keep the water at normal tank temperatures.</b>
3) Don't introduce lots of calcium structures into the treatment tank. I would not, for example, coat the entire bottom with an inch of crushed coral. Keep the crushed coral in a small bowl if you need it to place the clam. This is because doxycycline binds weakly to calcium, and I can imagine that lots of calcium carbonate might bring up the calcium ion concentration in the water very slightly.
4) Change the water and the medication out each day. I would recommend you get the water from the main tank rather than make it fresh. Freshly-made saltwater tends to stress stuff out, and you don't want your clam to be more stressed than it already is.
5) When the clam is put back into the tank, introduce as little as possible water from the treatment tank into the main tank. Again, this antibiotic is poison to the bacteria responsible for your tank's biological filtration.
I hope this helps folks!