Im in desperate need, I dont know how to care for my fish

I need to know what to use to get rid of nitrite and nitrate and alkalinity.. also I made a mistake because the water was BAD I cleaned it 100% and the next day it all shot up to nitrite-80,nitrate-3.0,hardnes. 0,alkalinity- 80, ph-6.8 . I USE DRINKING WATER, so it's perfect. I also use start right, bullseye 7.0, ammo lock, and prime..they keep dying I have 3 guppys 1 cory 1 pleco & 1 red claw crab in a 5 gal tank. I'm planning on getting a 29 maby 20 gal tank soon also. ANY SUGGESTIONS?

First of all, your 5 gallon is overstocked. The common pleco will get to be 18" long and is a pooping machine. You will need a 55+ gallon tank for him, so I'd take him back to the store. Cories are very social fish and need to be in groups of 4+. I'd take him back too, then when you get a bigger tank, you can get a nice group of cories. A few guppies should be ok in a 5 gallon. I don't know anything about the red crab, so I'll let someone else answer that part.

Did you cycle your tank before adding your fish? It's essential to do this before adding your fish to establish good bacteria. Before adding fish to a bigger tank, make sure it's cycled. Since you already have fish in a highly toxic environment, I recommend getting Bio-Spira (if you can find it). Otherwise, use Stability, it will do the same thing. It should help cycle your tank faster. Other products that claim to do the same thing are not as good. Do several water changes of about 30% every 2-3 days until your nitrites are under control (nitrites and ammonia need to be zero, nitrates should be under 20.) Make sure you add dechlorinator each time you add new water. I wouldn't add the pH adjuster, chemically changing pH can lead to pH swings that can kill your fish. You're better off keeping a steady pH that is slightly out of ideal range, most fish will adjust. Also, a weekly 20% water change will help keep your tank healthy once you get your nitrites under control.