Hi -
I'm so sorry for your losses, but hopefully with the help and support of the club you'll be able to overcome this and be well on your way to a thriving tank very soon!
I have to second the comments so far on your acclimation. Although I don't know if acclimation could be your problem if the fish were able to last 2-3 weeks, the method you posted might be a bit too quick. In my experience, fish stores tend to run their fish tanks' salinity on the low side, like in the 1.015 area. Some of them do this on purpose because they think it helps prevent parasites on the fish, but some do this accidentally because they're not paying very good attention.
Adjusting to higher salinity water is very stressful on fish (much more stressful than adjusting to lower salinity water) and it has to be done slowly; some people say no more than 0.005 SG per day. You don't say what your water's salinity is, but say it's 1.025, this means it would take two days to acclimate your fish, using this rule-of-thumb. In my experience, your acclimation doesn't need to be this deliberate, but it certainly stresses the importance of slow acclimation.
This won't help now, but next time you buy a fish, test the bag water's salinity first. This way you have an idea of how careful you need to be in acclimation. In my experience, I've been able to safely acclimate a fish from 1.015 to 1.026 in an 8 hour period, using drip acclimation. I dump the bag water along with the fish in a styrofoam cooler, then run some airline tubing from my tank down to the cooler. Start a siphon and tie a knot in the tubing to slow the flow to one or two drips per second. This will ensure the acclimation goes nice and slow.
Hope this helps, and good luck!
