Salt will not influence your hardness one way or the other. Sea water is very alkaline, but the elements which buffer it so high are things like calcium and magnesium.

Salt will significantly (sometimes hugely) raise the TDS or total dissolved solids in a tank though. All sodium chloride and no other elements can be fatal both to freshwater and - I would guess - even marine fishes after a time.

TDS is often measured by meters which really are measuring the electrical conductivity of water. There is a rough correlation between the two.

Now I've recently learned that there are a number of items in the water which are neutrally charged and not easily measured short of using a few 5,000 USD gismos to do that. Even some fish wastes may not be measurable! At least they don't contribute to hardness or TSD. (I can hear Guppygirl saying, "Go get an ammonia kit.")

I guess that is one of the reasons why we just need to keep changing water. There are probably always going to be some variables we will either not know about or not be able to measure.

If that wildly fluctuating tank, which has a pH a degree higher late in the day than the pH at night or in the early morning, has a lot of growing plants in it, don't worry. When the photosynthesis process is really cooking during the day, the plants will take a lot of CO2 out of the tank. At night, they may actually release a little CO2 and the pH will drop. This is not uncommon in ponds and aquaria with a robust plant population. I believe that some small bodies of water in the wild may do that too.

Congratulations on the flowering Aponogetons. You may find that they are NOT self fertilizing. Some books suggest taking a small paintbrush and rubbing one, than the other inflorescence, er bloom.

That never worked for my Aponogetons and me. I did discover that by rather inelegantly rubbing two blooms together they would fertilize one another. I had crooked blooms, but fertile seeds. ;)

Is it your wet (or wetter) season now? Different species respond differently to annual cycles.

Karl Rataj's book published in the 1970s by TFH has some interesting info on raising them. Christel Kasselmann has quite an essay on types of habitats and reproduction.

By the way, if your local Aponogeton spike throws up two separate flower spikes, that local population is probably an exotic from Madagascar. Single spikes are found all over Asia and, presumably, Australia. However there is a Vietnamese species with a double bloom. (Rats, so much to learn.)

South Africa's Apon. distachyos, which has become naturalized all over the world, has quite an elaborate multiple bloom. I think it was listed as an exotic in Oz (and California too).
By the way, Kasselmann's book features a magnificent shot of Australia's Apon. elongatus var latifolia. It is listed as from the North, NE and Eastern parts of Australia, so it might be your baby. :)

If you could get a couple photos of the blooms, that might enable you to identify them.

And they are a heck of a lot easier to photograph than the guppies. ;)