When was the water tested? I'd be extremely surprised if all is now as it should be as the tank is very overstocked and it's likely ammonia or nitrite have risen as a consequence, hence the deaths. Unfortunately, it isn't big enough for even one goldfish .... see this article for reasons why: https://www.fishkeeping.co.uk/articles ... -size-life-expectancy.htm
The best thing to do is return them to the shop and get something more suited to a small tank.....maybe White Cloud Mountain Minnows, though the tank being tall rather than long is not ideal for them either. If it was mine I'd add a heater and go for either a single male Betta splendens or a shoal of tiny tropicals such as mosquito rasbora or celestial pearl danios, depending on how hard your water is.
You can continue adding a small amount of bicarb with your weekly water changes to maintain the kH in the tank, or if the fish you have in mind need harder water than you have, you can dose with a product such as JBL Aquadur to raise both kH and gH as required.
You should be always doing at least 25% a week as regular maintenance. If you have ammonia in the water you need to do more, any time it is present. You have said nitrite is 20; do you mean nitrate? If it's nitrate that's OK but if you really have nitrite at that level you need to be changing almost all of the water to get it down to close to 0.
Re: Cycling and how to avoid New Tank Syndrome - by Alien Anna
Nitrate tests can be thrown off by high nitrite so I would think the result from the NT labs kit is most accurate. I would do a large 50% or more water change to knock the nitrite down and then continue. I would expect the cycle to complete in the next week or so.
There aren't any typical bottom-feeding fish that do well in hard water, there is the Tanganyikan fish, Synodontis lucipinnis......a group of those would be OK with mollies in your water and that size tank. You could have six or so mollies.
You need first to find out how hard your tap water is. You can get a good idea from your supplier's website where you can put in your postcode and get a water report including hardness. That will determine which fish you can keep as guppies and mollies like hard water and cories and rams like soft water.
I usually sterilise plants with potassium permanganate - just enough in a bucket to turn the water a pale pink. Soak for about 10 minutes and rinse thoroughly afterward. The other option is a bleach solution. Use cheap, thin household bleach with no other additives and dilute 1:20 with water. Soak plants for 10 minutes then rinse and soak in a bucket with a heavy dose of dechlorinator.