I think what is happening is that you put two delicate species into an uncycled tank. You say your nitrite is fine but you don't give the ammonia level.
Ammonia is the first thing that rises in an uncycled tank. You have to wait for ammonia eating bacteria to grow and turn ammonia into nitrite before you see any nitrite, which is why your reading for that is good.
What you need to do first is a big water change. At least 75%, making sure that with such a big volume the new water is the same temp as the tank water. And the same every day until you can measure the ammonia level.
Then read
this which will explain what you need to do to save the rest of your fish. When you kept fish years ago, depending on how long ago, the nitrogen cycle may not have been understood. Our understanding of what happens in fish tanks has come on enormously in the last decade or two.
Then buy an ammonia tester asap. You need to know how high your ammonia level is, and how frequent and large your water changes need to be.
And get a new heater that won't push the temperature that high!
If you are prepared to put in the work a fish-in cycle needs, you should get through this without more losses. However, the soft water may not help with your guppies; you may find that more of them succumb. If this does happen think about replacing them with soft water fish - once the tank is cycled.
Neons and guppies used to be hardy fish, but not any more. Neons are bred in fish farms. The conditions they are kept in are often less than ideal, and breeders tend to use any fish to breed whether they are healthy, robust specimens or not. Guppies are also big business, and they have the added disadvantage of inbreeding to get those fancy colours, which also perpetuates 'weak' genes.
Are
these your albino sharks? How long is your tank? 150 litres is a fair volume but it is length that matters with these fish. If this is what your albino sharks are, and your tank is less than 120cm (4 feet) long I suggest you return them to the shop.