fish die with sorts of reason, it could be water para, foreign object getting into the tank without we knowing it, parasites and etc etc. in ur case, since the guy of the shop not letting u to touch ur own tank then it is better for u to call them up to chk on the tank. remember bro u r paying them to keep the tank healthy and not paying them to make urself stress due of losing ls every single day. and i noe tat the contract norm include replacing of ls. it is best tat u can ask them basic test kit for ur aquarium. get in action b4 any worst scenerio happen to ur tank.
i werk for aquatic co. b4 tat is wat i usually advise my cust tat we do the necessary maintenance and it's the tank owner responsible to take care and monitor day by day the progress of their tank.if something u find out tat not doing very well, u should ring them asap.