Really sorry to hear about your fish, total loss or by chance any make it? Some fish can handle temperature extremes better than others especially if its a gradual change. I doubt Tetra will respond. I lost a bunch of fish using a product they make called RO Right. I wrote them an email and didn't even get a automated response. When it comes to filters, and heaters a friend once told me you need to buy the best you can afford. The initial expense will pay for itself.
Work In Progress