Oh, sorry, I haven't done any water testing in years. I remember that my water is pretty hard. I don't bother with trying to adjust parameters -- I just keep it clean.
I would say yours are older than 6 months if they were already 1.5 inches when you got them. I'd guess they are closer to a year or more. But the rate of growth really varies. The young ones that I raise in the pleco tank grow much more quickly than any I move into other tanks. They get tons to eat and huge water changes in the pleco tank.
If you want to give the fry the best chance of survival, a tank of their own is a good idea. Feed them a good staple food and some fresh veggies or try the canned French style green beans. Mine insist on French-style! They will like a nice piece of driftwood as well. Also a variety of cave styles and sizes to choose from. Most of mine like small and tight fitting caves, like those stones that I have pictured earlier. Those stones are even smaller than the small size cichlid stones that can be ordered online. I also have a log-style cave that I got from Aquarium Coop that gets used regularly.
I've noticed in the past that a particularly large water change or a water change that lowers the water temperature just a little bit will bring on spawning.
My big male took a few spawns to figure out how to keep the eggs in his log. He kept fanning them right out the big opening. The males that use the stones have never had that problem, I guess because there is a lip that keeps them inside better? Not sure about that.
The eggs don't take long to turn into little plecos. My males keep them inside a few days after the egg sacks seem to be absorbed, then one day there will be babies all over the place!
The first picture is from the day I found the eggs kicked out of the log. The last picture was only 5 days later.
Cute little babies on a suction clip that I put a few green bean strips on.
Good food, clean water and you should see babies before long.