You can save the money for a test for GH/KH if you can ascertain these numbers from your water authority. Check their website, they may have water data. GH is general hardness also called total hardness; it is primarily tyhe level of dissolved "hard" minerals especially calcium and magnesium that are in the water naturally. These are not going to change much in the aquarium. The p|H is related, but the GH is the important parameter you need to know, as it does without question affect the internal physiology and metabolism of fish.
You also need a test for ammonia, nitrite and nitrate. As has already been mentioned, cycling is probably an issue here. Mollies are highly sensitive to ammonia and nitrite, but it is the smaller fish with a higher metabolic rate that usually succumb first, as here with the neons.
As you earlier said, sort all this out before acquiring any new fish. You may or may not have to re-home some you have, depending upon the parameters. And the cycling issue is paramount. Someone mentioned cories, they need a sand substrate, F.Y.I.
You also need a test for ammonia, nitrite and nitrate. As has already been mentioned, cycling is probably an issue here. Mollies are highly sensitive to ammonia and nitrite, but it is the smaller fish with a higher metabolic rate that usually succumb first, as here with the neons.
As you earlier said, sort all this out before acquiring any new fish. You may or may not have to re-home some you have, depending upon the parameters. And the cycling issue is paramount. Someone mentioned cories, they need a sand substrate, F.Y.I.
