OP some very knowledgeable, well respected people on here are trying to help you out, but you are being very rude to them. I’m not sure if the LFS showed you or told you what the ammonia levels were on their test when they said ammonia levels killed the fish but I would look into that. If they are claiming ammonia levels and not reimbursing you for the fish, then I would want to see the proof.
Determining the cause of death could change how you proceed. Test strips in general are just not reliable. Looking at your ammonia test, it’s not the most trustworthy test kit, but I would lean towards believing it’s a disease issue but definitely curious what the LFS found. That said, every LFS is going to be different and they could be running unreliable tests themselves. If you can determine that the cause was not water quality related and was in fact disease related, then the fallow period mentioned above is the way forward.
I am like you in the sense that I don’t have a QT tank and have 1 single small display tank, but I understand the risks. I’ve been lucky so far, but if I introduce a single invert or fish that has disease, I run the risk of losing everything so I am going to be very cautious going forward. Ordering from a pre QTed source online is a good option in our case.
Either way, I would recommend investing in some good test kits soon. To me being confident in my testing equipment is very important. I recently had a little ammonia scare due to some snail deaths in my small tank and now keep an in tank ammonia monitor in there. Long term, you probably want to have test kits available for Salinity, PH, Alkalinity/DKH, Nitrates/NO3, Phosphates, Calcium, and Magnesium. I suppose that depends on how seriously you want to take keeping your coral alive and healthy, but at the very least, I think you should have a good test kit for Salinity, Nitrates, and Phosphates. I use a Milwaukee refractometer for salinity. I use Salifert for PH,DKH,Calcium,Magnesium and Hannah checkers for Phosphates (PPB) and Nitrates (high range). To me the salifert kits weren’t cutting it on those 2 and I wanted more precision. There are many other options, but that’s just what I’ve been using.
Sure there are people out there that have had success using tap water instead of RODI and not testing anything but I can’t imagine investing the kind of time and money it takes to get one of these tanks going without setting myself up for success by researching a ton and using quality tools to make sure I am on track.