Very sick female guppy

As the title say I have a female fancy guppy that is extremely sick. She was ok when I got her(early January) but not too long after she was in the tank her gills(and the other guppies) puffed up and were red. I didn't know at that time but I hadn't cycled my tank properly and after I realized it I though it was the ammonia. Most of the guppies died except her and one other one that is doing much better than she.
She was(still is) very skinny after she dropped her fry not long after I got her. They were all healthy minus being a little early. She still ate and breathed just fine until recently. The tank just finished it's cycled it's a 45 gallon tall with two large sponge filters and a aqua clear 75 hob filter(it might be a 50 instead but I'm not certain)
The tanks parameters are
ph: 8.0
ammonia: .25-.50 the colors are really close so might be in the middle still too high I know but I've started to feed much less since I checked so it could be lower
nitrite: .20
nitrate: 20
I have 5 corys, 1 male betta(in a breeding net), about 30 fry most will be going to a different tank, 2(now one) female guppies, one clown pleco, two amono shrimp and one cherry shrimp.
I put her in a 3 gallon hospital tank with airration and salt(1/2 a teaspoon per gallon) the salt is aquarium salt by api
two days ago she went down hill and stayed at the bottom was breathing really hard and wouldn't eat, she also seemed to loose more weight. I had put some pimafix the the water to fix some mouth fungus that popped up in the tank due to my own mistake by not quarinteing some new female guppies(they all died from it and birth complications)
the rest of my fish seem fine minus some of them seem to be itching and have started to get the red gills too

She still tries to swim but just goes in circles
She wasn't doing this last night

Are the test results from the main tank or from the quarantine? Either way, you need to be doing some major water changes in that tank to get the nitrites and ammonia as close to zero as possible. Those levels are deadly, in a cycled tank both should be zero at all times. This is what is causing all the problems. When did you test the water last? Might want to do it again for both tanks and post the results.

Water changes are your best bet for the ill female and all the others. If you can get her back to swimming properly she can eat and hopefully gain weight. I'm concerned she also has internal parasites, which are basically absorbing all the nutrients from her. Notice any white, stringy poo or fish spitting food back out? That's a sign of parasites. To treat that I'd go with Prazipro, but first you must get the tank cycled. No point in adding meds when you will have to do a water change the next day.

1 male betta(in a breeding net),

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Why is the Betta in a breeding net? That is very stressful on him.

I have 5 corys,

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With a PH of 8 this could also be a problem.

Basically your water is toxic, You need to do some mayor water changes, I would do 70% right now,

What water conditioner are you using?

Can you tell me how you maintain the tank please?

How long has the tank been set up?
How did you cycle it?