That's surprising that the mollies didn't make it. The sulfur smell is probably anaerobic hydrogen sulfide coming from somewhere. There are some African lake cichlids that live in rock water. Make sure to get the kinds that don't eat plants.

Shrimp would be fish/loach food.

 

The water is slightly yellow because of all the iron, and smells of sulphur

Your water is acidic and has no KH sulphur in the water means high sulfates and acid in the aquifer. That acid will dissolve a lot of stuff out of the rock including calcium and magnesium and iron. High sulfates in the water might not be safe for fish. I don't know of any fish that live in in the wild in such water. Also highiron can cause gill problems and could kill fish. what is your PH.

 

My PH is 8.3 and super steady. I am taking the water straight from the well, but I could take it after the water softener...? 😬

 

Water softener replaces Calcium and magnesium with sodium. Typically The end result is water with Zero very low GH and a high KH value ( due to sodium bicarbonate). However with your high sulfur levels you might get a low GH and a low Kh with most of the sodium in the form of sodium sulfate (Not sure about this,I haven't seen this type of water problem). The high sodium levels are probably not good for fish and you would have to add a GH booster to get enough calcium and magnesium for plants and fish. I don't think using your well water or the softened water is a good idea.

I think you should connect a RO system to your softened water. RO water is about as pure as rain water. You could then add a GH booster and fertilizer the water. For PH control add some crushed coral to your filter. Plant growth with GH boosters tends to push the PH down. The crushed coral would neutralize the PH and keep it near 7. that way you could use any fish you want.

Sorry, this is not going to be easy for you. I think it is best for you can keep your tanks as is for now. work on getting a RO until installed.and then slowly replace the hard tank water with RO. You could even mixing a small amount of well water (A free GH booster) to get the GH up without using a purchased GH booster. This would've you the greatest flexibility in selecting fish.

 

Do we have an update on this tank yet? I'm interested to see what fish if any you try, and how they do.

Is it rural where you live or are you in the city? If you are rural or the countryside is not too far away (you have a well so I suspect you are living in the country,) I was thinking you might try collecting some local minnows, croppie or other fish, snails and crayfish who currently live in this water and are already acclimated to it. Of course you'd need to 'clean them up,' treat them for any parasites or diseases they might be harboring before dumping into your beautiful tank...but that might make an interesting sort of biotope project, a representation of what lives in your water naturally. What do you think?