Why do my neon tetras keep dying?

You could be just unlucky and are getting poor fish, change the supplier, however, it could well be the temp mine are usually kept at around 84-86f, hardness and pH are a little above what i'd describe as optimum but that should have little effect but combined with the temp may be taxing. Whats the water flow like while they're a nimble fish they tend to hang out in slow areas with root structures overhanging so a strong current can cause problems.