The condition referred to as ''Skinny Disease" is often a combination of problems.
The fish may be infested with internal parasites and also have some bacterial issues.
Levamisol is good as a boost to their immune system and very good against internal parasites. There are several types of internal parasites, and no one medicine gets them all, but Levamisole does eliminate most. I have heard that some wormers just stun the worms, the worms are expelled, but still alive, and can reinfest the fish. Be sure to vacuum the floor of the tank several times. The other aspect of controlling this type of pest is that most wormers will only kill the adults. When the eggs hatch you need to treat again. If you can ID the pest you may be able to use the optimum active ingredient and timing to target that specific pest.
To attack the bacterial issue use a medicine against Gram negative bacteria. Most infections in fish tend to be Gram negative bacteria.
Here is a general schedule. Read the labels on the products you are using and adjust the schedule accordingly.
1) Wormer. Usually keep it in the water for 3 days. This may mean water changes and re-dosing.
2) 50% water change, emphasizing vacuuming the floor of the tank, and 24 hours with activated carbon.
3) Antibiotic. Usually a 5 day treatment.
4) 50% water change, emphasizing vacuuming the floor of the tank, and 24 hours with activated carbon.
5) Wormer. Usually keep it in the water for 3 days. This may mean water changes and re-dosing.
6) 50% water change, emphasizing vacuuming the floor of the tank, and 24 hours with activated carbon.
7) Antibiotic. Usually a 5 day treatment.
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50% water change, emphasizing vacuuming the floor of the tank, and 24 hours with activated carbon.
9) Wormer. Usually keep it in the water for 3 days. This may mean water changes and re-dosing.