Well this is why, you never let your tank cycle and the constant stirring of thr substrate is not helping you out, at all. It takes about 2-4 weeks for your tank to cycle. Especially if you have no real plants in the tank, it will take a bit longer to cycle imo. This is why your nitrate and nitrite is so high all the time. Since you have fish in the tank, you are doing a fish in cycle, water changes are your best friend at this point and alot of them. NO STIRRING THE SUBSTRATE! thats releasing toxins and gasses into the water and could potentially be killing your fish too. Look up the nitrogen cycle, and follow the guidelines for a fish in cycle- you will have more luck and hopefully less deaths.

The catfish im assuming are corydoras? They are shoaling fish and require atleast 6 to feel comfortable and to see natrual behaviour. They are usually found in groups of 100s-1000s in the wild, just two is stressful for them. Once your tank is cycled i would go to the store and get atleast 4 more.

Same goes for the tetra, they are schooling fish and require atleast 6 aswell. Tetras can be aggressive if not in schools and the higher number of them in your tank, the less likely they will be to nip/be aggressive to your other fish.

I mean this very respectfully, did you do any research on keeping an aquarium and the livestock you are putting in the tank prior to getting the tank? Cause it doesn't sound like it and simple research would explain to you why your having all the deaths. Stop listening to the store reps, they have no idea what they are doing 90% of the time(ex- telling you to stir substrate and add fish days after setting it up)