As I mentioned before
DON'T PANIC. (Those were meant to be large friendly letters
)
Those tetra strips continue to change colour after a minute - so be sure to take the photo after exactly 60 seconds and hold it against the scale on the tube. Preferably do this in sunlight or under an LED light. The good news is that if your fish are breeding they are essentially healthy.
I kept fish in very hard water with high nitrates for over 10 years before I realised that this was an issue. Your fish already have an advantage over mine because I had soft water fish - at least you have the right type of fish. The result of this was that my fish did not live as long as they could have, which I only discovered by accident speaking to a fishkeeper who kept similar fish to me. The fish simply died of old age and I never had any disease outbreaks. This is achievable if you keep up with regular maintenance: Water changes, cleaning filters, not overfeeding and having lots of plants. Now one advantage of keeping livebearers / hard water fish is that you can expect a constant supply of new fish. My tetras would not breed in that water so every 2-3 years I had to buy lots of fish as my stocks gradually diminished. So
do nothing is an option.
I do not say this because I am callous. The issue is solveable but is likely to be both expensive and time consuming. If you can provide the right conditions your fish will live "normal" lives, but just shorter than they could have done. These would still be better off than:
- those being kept in soft water tanks (and many people do this)
- those kept in badly maintained tanks (and many people do this)
Like it or not your local fish store is going to keep selling them and they will end up in water just like yours. At least you are making the effort to understand their needs and providing the best environment you can.
So lets get the numbers, and right now don't do anything other than good maintenance. We don't want to stress your expectant mothers by changing their environment right now. Once your fry have been safely delivered and grown big enough not to be eaten you can look at options if you choose to - and you confirm that your water does have high nitrates.