I think my fish is sick but I don't know what exactly.

yesterday I noticed her tail was drooping and she wasn't moving as much. Today it got worse, she spent the morning hiding until i coaxed her out. Would only eat a few bites of an algae wafer. Tail drooping alot, white stringy poop, mostly just stays on the surface not moving much. Her movement is altered, can't swim well. I only have melafix plus some ich and fluke medicine. I don't know how to treat her

This is a 30g, 10% water changes every week. I only add API liquid carbon, conditioner and some seachem equilbrium since my water is soft. ammonia 0, nitrite not yet tested, nitrate 12.5mg/l, ph usually 7.8-8.0. There is another swordtail and guppy living in the tank. they are acting normally.

I think what you might be experiencing in the fish is a bacterial infection. If you are only changing 10% of the water weekly you are not going to be diluting the bacteria present in the water, glass, filter, filtration media and so forth to an acceptable level. I’m not too familiar with the species of fish that you have but from the 2nd photo down it looks like it may have pop-eye.

I suggest you obtain an anti-bacterial named Waterlife Myxazin or similar to treat for a bacterial infection and to replace 80% of the water. Use some of the old tank water to try to remove dirt from the filter media. I also recommend giving the fish a mix of vegetable and protein foods (not just algae wafers) to ensure it stays healthy.

While it is not eating its regular foods, I recommend offering it blood worms until it starts to eat the usual things that you give it to eat.

don't add medications unless you know what the problem is.

check the ammonia, nitrite, nitrate, pH and GH.
most fish health issues are caused by poor water quality.
the first thing you do when a fish is sick, is test the water and do a big water change and gravel clean the substrate.