I will put my 2 pence worth in. I am by no means as knowledgeable as some of peers on here, but a couple of things strike me.
The main thing is a lack of a testing kit. I know you say they are expensive, but it’s relative and they last a long time, being able to get hundred of tests out of a kit. There isn’t just API master kit, have a look for NT Labs. That has your ammonia, nitrite, nitrate, pH, GH (general hardness) and Kh (carbonate hardness). That covers you. I have an addition oxygen tester from O2. It might be better to save and buy a kit so you can check your water as soon as there is an issue. Ultimately, it could save you money on fish if they are dying. Better for them, and better for your pocket.
Can I count 8 fish in there? That is a lot for a small tank. Especially the ones you have. They are heavy waste producers and your water quality will deteriorate quickly, which makes me think your water changes of 8-10 days is not sufficient. Especially if you can’t test it. Any way you can take the fish back, perhaps look at recycling the tank? You said you give everything a good clean by taking everything out. The beneficial bacteria might be being washed away so you end up doing lots of fish in cycles.
this is all friendly advice and you will probably read on other posts that knowledge is lacking in LFS’. They want your money so happy to sell you RO water, buy fish and test your water. Guaranteed income! Slow down a bit, listen to the experienced members here as they are an absolute fountain of knowledge and get your water sorted. That is what your fish live in after all. They have no control over that habitat, you do.
Thats enough rabbling from me! Good luck and don’t be afraid to ask questions. That’s the only way you will gain knowledge and experience!