Let's back up here a moment. Spawning a species is relatively simple, if that is all you want to achieve. If however you want fry from the spawn, that is a very different thing. You might be able to get neon tetra to spawn in their present tank, but ytou will almost never see fry because they will eat the eggs, or any other fish will eat the eggs, before they can even hatch. That is why others are recommending separate tanks, assuming you want fry.
Your GH is 159 ppm (ppm = mg/l) which equates to 8 dGH. That is moderately soft/moderately hard water. Most soft water fish will be fine in this, neons certainly will. However, I do not know if they will spawn, or if the eggs would hatch. I have had various characin species spawn over the years because I do now and then see a fry. Various tetras (never the
Parachierodon species though), pencilfish, Corydoras, cyprinids, cichlids...but I let nature do its thing. Most of the eggs obviously get eaten. But I suspect one reason there is som much spawning activity is because I am providing the fish with exactly the water parameters they "expect," with zero GH and KH and a quite acidic pH around or below 5. [And I never add substances except for conditioner at water changes and minimum plant fertilizer.]
Neon tetra are raised in outdoor ponds in SE Asia, Florida (I think this is still the case), and the water is probably on the very soft side though I have no direct information. If you want certain success with spawning neons, here is the method SF provides:
You’ll need to set up a dedicated tank if you want to raise decent numbers of fry. This should be very dimly lit and contain clumps of fine-leaved plants such as java moss or spawning mops, to give the fish somewhere to deposit their eggs.
Alternatively you could cover the base of the tank with some kind of mesh. This should be of a large enough grade so that the eggs can fall through it, but small enough so that the adults cannot reach them.
The water should be soft and acidic in the range pH 5.5-6.5, gH 1-5, with a temperature of around 80-84°F.
A small air-powered sponge filter bubbling away very gently is all that is needed in terms of filtration.
It can be spawned in a group, with half a dozen specimens of each sex being a good number. Condition these with plenty of small live foods and spawning shouldn’t present too many problems.
The adults can be removed once eggs are noticed, or in a very heavily planted tank left
in situ and
fry siphoned from the tank as they’re noticed.
In terms of productivity, it’s best spawned in pairs. Under this technique the fish are conditioned in male and female groups in separate tanks, or via the use of a tank divider.
When the females are noticeably full of eggs and the males are displaying their best colours, select the fattest female and best-coloured male and transfer them to the spawning tank in the evening.
They should spawn the following morning. If no eggs have appeared after a couple of days remove them and try a different pair.
In either situation the adults will eat the eggs given the chance and should be removed as soon as eggs are noticed. These will hatch in 24-36 hours, with the fry becoming free swimming a 3-4 days later.
They should be fed on an infusoria–type food for the first few days, until they’re large enough to accept microworm or brine shrimp nauplii.
The eggs and fry are light sensitive in the early stages of life and the tank should be kept in darkness if possible.
