Help needed, fish keeping is proving tough

[LL82]

Newbie here please be gentle. We bought a 28L fish tank for our son’s seventh birthday although we set it up several weeks in advance we did not fully understand the cycling process. Initially we got our fish from pets at home (possibly biggest mistake here). We had 2 platys 2 danios and 4 minnows, within a few weeks the minnows all died. We regularly got the water tested at a reputable fish shop until it was good enough to add more fish. In the last two weeks we had the original 2 platys 2 danios (we lost 1 and replaced it) and added 2 minnows - all from the shops which has been testing our water. We were told the water was good. However one of the platys became lethargic and today we lost it 😢 does anyone have any advice? We have been really enjoying having the fish but it is much more complicated than we thought. We think we are doing okay and then something happens and we lose another fish and it really is sad.

I think you had too many fish and they weren't right for your tank. 28 litres is very small. Did the fish shop give you any advice on stocking it? Also some fish prefer to be in groups of at least 6 as they are community shoaling fish. You also need to think about planting, temperature and water flow.
There are sites that advise on what fish are suitable for your size of tank and how many. For example https://completecalculators.com/calculators/aquarium/fish-stocking-calculator
Or
https://www.aquariumstockingcalculator.com/
although I haven't used them as I generally discuss with the shop staff for advice as a good supplier should also be able to help. There are also facebook groups that msy help.

If the shop is testing with strips then they are not always accurate. I recommend getting the api master test kit so you can keep an eye on things. It is a complicated hobby, but once you get things set up right hopefully it will be worth it!