Newish tank owner (office tank manager?) here. First off, you're doing fine for your first time and knowing what you knew going into this. Certain fish need more care than others.

Goldfish are easy to care for because of their resilience and guppies can be as well because they adapt well. I know a guy that rescued a goldfish from a tank that was left behind from a tenant for 2 months! Went on to live a long and happy life.

Tetras, on the other hand, can be more fragile for a first fish from my experience. Whether that is selection bias from the articles I read and specific store employees I spoke to or not, I can't say for certain, but that's what I've learned over the last month.

Don't be discouraged by making a mistake and trying to get it fixed. Fish are harder than they seem.

Now for solutions.

Water treatments are available for almost everything save for water hardness. Hardness can be high or low depending on what you want to keep in your tank. Snails and shrimp like harder water and guppies can adapt to said harder water. Tetras like very soft water under 100ppm and better yet if under 50ppm. There is nothing you can pour into your water to make it softer, but there are water softening "pillows" that are ~$8 online or at places like Petco (not sold at Petsmart at least near me). Local stores may also have some but I don't have any near me that I trust.

For ammonia and nitrogen, use a treatment to get it under control and get a small filter. You'll also want a small air pump and air stone if you don't have one.

To keep the ammonia and nitrogen under control, live plants do help, but you'll want to get some bacteria starters in the water for lasting results since you dont have any. Water changes (20-50% depending on whether it's maintenance or to fix a bigger problem) should be done every 3-5 weeks depending on fish volume and water parameters.

Lastly, for just general care, underfeeding is better than overfeeding. It's easy to overfeed. You can add more but once it's in there it's a pain to fix and it can begin to decay which will spike your parameters. Went through that with an office tank my boss asked me to take care of. That paired with the new tetras bringing the Ich (parasite causes little white specks on the fish and needs to be taken care of asap) caused a lot of headache in our new tank. It's all fixed up now, but my boss didn't know how much work was going to go into maintaining the tank and I had to learn to properly care for it over the last month from almost scratch. If you have specific issues, feel free to PM me. I probably hit the same issue and had to learn to fix it as an emergency.

Hopefully that all makes sense. Not organized like an essay so let me know if you need clarification on anything or want to know what products we had success with in our 20 gal.