I would not use that general tonic. It can claim whatever it wants. The ingredients are mainly stuff that would not help fin rot and the cocktail is toxic and harsh.
I would really make sure that nothing is getting in the tank and this is really his poo before treating him. 100% water change, bare bottom, and away from the dog.. Here is a picture of something that you might see with an internal infection: http://img.photobucket.com/albums/v1...harliePoop.jpg
See the white stuff?
Assuming this really is what you're seeing..
Internal infection is much more pressing, and really it could be bacterial or parasite in nature but being as skinny is he is I'd go the parasite route first. Just like the fin rot, frequent water changes are required here. The internal infection requires epsom salts - dosed like I wrote out back on the first page, and treatment of feeding medicated food or a medicated bath for 2-4 weeks.
As a feed: You need a good soaking pellet with lots of wheat - you have one. You can also use garlic guard or vitachem fresh water to help boost his immune system and be more likely to eat the pellets with the meds, but you don't have to. You can also use some dechlorinated/treated water. You also need a source of metronidazole. You can use API's general cure if you can't find pure metro. To mix the feed you take some of the water into a cup and one scoop of the meds and mix it up as best you can. Then you put the pellets in the treated liquid and soak them for 10-15min. You feed two of these twice a day - 2 am and 2 pm. I suggest mixing this up fresh every time you feed, but you have to mix up fresh at least once a day. If you wish to save it for the second feeding, cover it and put it in the fridge. Be sure to bring it back to room temp before feeding the second time.
You need to feed this a minimum
of two weeks and at least
a week past
when he seems 100%. I would feed a minimum of 3 weeks personally, and many will feed a whole month to be safe. The reason is the chance of relapse is really high in these when treatment is not continued long enough.
As a bath: I haven't had much luck with baths but I have seen some luck with Jungle Parasite Clear. Follow the instructions on the box for a 3 week treatment. This is a lot harsher than the feed method and I personally wouldn't use it because of how harsh it is and because it's just not as effective as making a feed. Tetra Parasite Guard seems to be a similar product, but is again very harsh.