Answer:
First off, since your nitrate is high, you need to do a large water change IMMEDIATELY, not just a little bit over time. Do a 50% water change ASAP so you won't sicken any other fish or cause your female guppy to get worse than she already is. High levels of ammonia, nitrite, and/or nitrate can kill your fish overnight, or stress them to the point that their immune systems don't function properly and they will succumb to various bacterial, fungal, or even viral infections.
Secondly, it sounds like your female guppy as a swim bladder disorder. This can be a result of overfeeding, constipation, bacterial infection (rare), or a birth defect (very rare, and obviously not the cause, or your female would have always exhibited this odd behavior). Since you said the other guppies are picking at her, you need to remove her from the tank and place her in a hospital tank IMMEDIATELY, as this will only exacerbate the female's problems.
The next thing you need to do is try to figure out the cause of the SBD. How much food do you feed your fish? If you feed them a lot (remember, a fish's stomach is roughly the size of it's eye, and overfeeding will cause the stomach to stretch and constrict the swim bladder, which maintains a fish's buoyancy), then you need to cut back on the amount of food your feeding. As for the female with SBD, even if you don't think overfeeding is the cause, it would be helpful to fast her for 24-48 hours. Most fish can go over a week without eating, so don't think you're doing her any harm. If she has an excess amount of food in her stomach, or may even be constipated (which is another cause of SBD), then fasting your female will help clear out her digestive system and hopefully correct her SBD.
If you think your guppy is constipated, try to remember if she's pooped lately. Her abdomen may also be abnormally swollen also from the unpassed waste. To "cure" constipation, you can do several things. One, like I said in the last paragraph, you can fast your female for 1-2 days. If she still hasn't passed any poop (this is easy to discover if you keep the bottom of your hospital tank bare - which you should) by then, I strongly recommend feeding either a blanched pea or live or frozen daphnia (NEVER freeze-dried daphnia - freeze dried foods swell up in the intestinal tract and would only exacerbate the issue). To fish, peas and daphnia (a water flea) are natural laxatives and clear out the digestive system in no time. I usually feed daphnia over peas simply because I have better luck with getting my fish to accept daphnia, especially bettas, although guppies are ominvores and rarely reject any sort of food.
If none of the above treatments work, it is possible your female guppy has a bacterial infection in her swim bladder. This is exceedingly rare and very difficult to treat, so do not automatically assume it is bacteria! This is your second-to-last resort at treating SBD. If you have good reason to believe your female has a bacterial infection, I would suggest using either tetracycline or furan 2. I have personally never had a SBD case that was caused by a bacterial infection, so I can't offer too many pointers here other than to use the medication as instructed.
Also, when attempting to cure SBD, it helps to add 1 tablespoon of salt per 10 gallons. However, a final and more dangerous way to cure swim bladder disorder is to give your female guppy an epsom salt bath. I strongly recommend doing this ONLY as a VERY last resort and when none of the above suggestions work. Also, some fish are unable to survive this treatment, mostly those who are weak to begin with, so I would proceed with MUCH caution. Anyway, here is a "recipe" and instructions I found on the internet (It was on a betta website, but this is a universal recipe for all types of fish, not just bettas):
"Setting up an Epsom Salt bath:
Add 1 Tbsp per gallon of Epsom Salt to some aged, temperature-adjusted (same temperature as the water the betta is already in) water.
Pour Epsom Salt solution into a container.
Place the Betta in the Epsom Salt bath for 15-20 minutes. While the Betta is having a bath, change the water in his tank so that he has nice, clean water to go back to afterwards. Make SURE that the water in all of these containers is the same temperature, we don't need the Betta getting another disease right now."
