I fried my fish...

Oh, what a terrible day!

I had a heater do the same thing many years ago. Lost a breeding pair of Firemouth cichlids and about a hundred of their fry. :sad

Yeah, I've had this happen to me in the past and lost fish from it. Some of the cheaper heaters that use a bimetallic strip to turn the heater off and on are very problematic when it comes to getting "stuck on." I would recommend a 50 Watt Ebo Jabber or Tronic brand heater. 50 watts may be a bit much for a 5 gal but you can't really find a lower wattage decent heater in my opinion.

Exactly, many times the lower wattage heaters are cheaply made which makes no sense considering most people with aquariums under 10 gallons are people new to the hobby...

...Anyway, this is why many aquarists dont recommend smaller tanks. They really are a pain to keep up with, including the accompanying accessories that comes with a small tank (heaters, filters, etc.)

And as for the pH drop, I know this happens a little as water quality worsens. When was the last time you did a partial water change on your 40 gallon? If over 2 weeks, this could be your problem. If not, have you tried testing the water that comes out of your tap? Sometimes water companies treat the water with different "ingredients" so to speak...or difference amounts anyway.

HTH

had it happen to me, went to about 95 degrees, lost a pair of kribs and asian bumble bee cat, luckly my gold serverum surrvied/ I still have him to this day and hes doing fine.