There are three (3) basic possibilities when it comes to an intestinal problem. This reference outlines the three nicely:
http://www.petsforum.com/personal/trevor-jones/internalinfections.html
Garlic is not a remedy. It has been suspected of helping in one case of internal parasites. The fish was fed solid chunks of it. Garlic juice does nothing in this case. I can elaborate on this if you want.
Sometimes an internally infected fish (bacteria or worms or parasites) will stop eating or slow its intake. But usually there are other signs, like loss of weight, ravenous eating but no weight gain, and/or strange excrement. Consider the possibilities by reading that article.
Whatever the internal problem, the fish needs the best water quality and the best diet AND supplements added to its diet.
For diet and supplement help, check this out:
http://www.reefcentral.com/forums/s...threadid=785228
If the fish isnââ"šÂ¬Ã¢"žÂ¢t eating then the only way to get medication inside of it is to put the fish in a quarantine tank and treat the water with chemicals that will kill the internal condition. The drug Praziquantel (a.k.a. droncit) will treat intestinal worms, Maracyn Two for Saltwater fish will treat an internal bacterial infection and metronizazole will treat dinoflagellate infections.
To be conservative: First check your source water for contaminants. Use only the best source water and make up a new batch of salt water. Now check that water for quality/contaminants. If it all passes, then do an 80% water change and see if the fish seems to 'perk up' (e.g., starts eating or eats more or swims around more in the open). If no response, treat the fish for worms, but also include a metronizazole treatment. If the fish definitely perks up (starts eating or eats more), then it maybe a combo of water conditions and bacteria. In this case, I'd treat the fish for an internal bacterial infection. Make sure this water change is performed with properly aged, properly mixed, and 'matched' saltwater
If the fish was fat and eating properly and being fed a proper diet (see above reference), with vitamin and fat supplements, it can live without eating for several weeks, providing all other environmental and water quality conditions are as they should be and the fish isnââ"šÂ¬Ã¢"žÂ¢t diseased. Praziquantel is best administered orally so see if you can get the fish eating.
Good luck!