What is the ammonia, nitrite, nitrate and pH of the aquarium water?
What sort of filter is on the tank?
How often and how do you clean the filter?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
---------------------
The fish is emaciated and could have intestinal worms and or gill flukes. It might also be starving and not getting enough food due to bullying. However, the tank looks big enough for the fish you have so bullying is unlikely. Starvation is a possibility.
What do you feed the fish and how often do you feed them?
---------------------
Feed them a variety of dry, frozen and live foods 3-5 times a day for a month. See if it helps.
Wipe the inside of the glass down with a clean sponge.
Do a 75% water change and gravel clean the substrate every day or every couple of days while feeding more often. This will keep the water cleaner and reduce other health issues occurring.
Clean the filter if it hasn't been done in the last 2 weeks. Established filters should be cleaned at least once a month.
Deworm the fish. See section 3 of the following link for deworming fish.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
www.fishforums.net