Fish keep dying face down?

[Sachiel]

So, it's been a while since I've had any fish. However, the last fish I had kept dying with their tails to the sky and their faces in the gravel.

I noticed this happened after I put my heater in. The heater is keeping the water in the right spot for tropical fish (had a betta) and it's functioning fine, so I have no idea if the heater caused it.

I've used this tank (5 gal "square" tank [it's rectangle]) for many fish in the past and they've always died "normally."

Right now I am letting my tank run with just the filter, heater and thermometer (and obviously water.) Nothing else is in there. I'm going to dump it out and rinse it out a few more times before letting it really cycle for a fish, but I'm wondering what may have caused them to die like that? Is it just the way the fish were "built?"

Even when I put my goldfish in there (after cleaning it out well because the betta had ich) it also died face down after having to swim in that position, or sideways like a flounder.

Should I remove my heater and replace it, or see if another fish does the same? I think two incidents is enough, but I want to find the problem.

Sorry for the wall of text lol

 

[Mumma.of.two]

Did you check the parameters at all?
How are you cycling the tanks?
Did the fish show and other signs of disease or poisoning like red gills, open sores, black marks, fuzz, cottony growths, spots, etc?