Red gills can be from parasites and/or water quality issues, you should treat the tank with "anti slime and velvet" by interpet as it'll also treat any parasites or gill flukes that are causing her gills to look sore.
The concave stomache is most likely either;
a. She gave birth to a lot of guppy fry and it has taken a lot out of her, making her ill and very thin.
b. Internal parasites- nasty, but needs to be treated. An inflamed or sore looking/red anus is a classic symptom of internal parasites.
c. Internal bacteria- white/stringy looking poop is a symptom of internal bacteria (can also be a symptom of internal parasites too), either cause a lot of weight loss in fish or bloating depending on what part of the fish body the internal bacteria is affecting etc.
d. Fish TB- fish having a curved/warped spine and/or flesh is a classic symptom of fish tb; there is no cure, so choosing a humane form of euthanasia to perform on the fish is the best course of action to prevent it spreading to other fish.
All of these things listed above can cause the fish to look very underweight/have a rather concave looking stomache, so its important to find out what exactly your fish has, treat it, and prevent it from happening again.
You shouldn't be using chemicals to take away the ammonia and things in the tank, these shouldn't be used in tank set ups which are established or at least cycling normally with a suitable filter for the tank- you need to learn more on the Nitrogen Cycle as i think you have a wrong understanding on how the water quality in your tank works.
Below is a very helpful link on explaining how the water quality in your tank works and so forth (i highly recommend you read it);
http
/www.fishforums.net/index.php?showtopic=10099
The only chemical you should be using in your tank is dechlorinator/water conditioner to remove the chlorine and heavy metals from the tap water in your tank- nothing else is needed. You shouldn't be replacing the filter sponges in your filter, you should only be roughly washing them out in old tank water from water changes when they become very dirty and the filters current starts to weaken (which for most people is every 1-3weeks). You should be doing a 30-40% water change with dechlorinator once a week in a normal and healthy tank set up

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If you are getting ammonia in your tank, then something is amiss which is causing it to cycle. The problem is ammonia removing chemicals can sometimes mess up water quality test kit results, so you need to stop using such chemicals and then test the water for ammonia, nitrites and nitrates.