Will my guppy survive?

Ok I just came back from vacation and I have not done a water change in about 8-9 days which I usually do weekly (20%). I have a 5 gallon tank with 1 male guppy, I neon tetra, and 1 angel catfish. The fish have been doing perfectly fine for 3 months now. My guppy is a VERY active fish. He does his mating dance for the tetra all day-everyday. He eats normally and is starting to recognize me. My mom was fish sitting for me and Im sure she over feed as I come home to find the black gravel practically covered in old fish food. I did a water Test and found the nitrates super high!!!!!! I did a 20% water change, added 1/2 teaspoon bacteria, 1/2 of nit out (the nitrites was stressful too) and added some water conditioner to the purified water I added to the tank. I also tried to scoop up as much left over food as possible. There is some algae starting to grow on the decor. His skin and tail looks fine, he just has his long beautiful tail (that he just grew) laying on the gravel while he paddles & wobble upward breathing normally. That was last night; this morning the gravel is a almost spotless thanks to my catfish who's a nighttime eater, and my guppy is doing the same behavior but when I put a pinch of food inside (I know it's bad but I was curious) he swam around eating crazy as usual and at the top chomping away, and afterwards went back to the bottom. I really love this crazy fish. I checked my water this morning and it noted -nitrate at the end of the safe zone @40-nitrite @ 1.0 stress area-soft water-alkalinity low around 40-and PH @neutral 6.8. What's wrong with him? Will he survive? What can I do?