You have probably already said it in the past but what kind of water do you have in your tank Biokid? If it is soft water with a neutral pH, that may be part of the problem. I find that livebearers thrive in my hard water with a high pH. It is one of the reasons that I focus mostly on livebearers. In my water, they are very easy to rear as long as I do occasional water changes and gravel cleans.
With my heavy plantings, I go more in the Walstad NPT direction than many people and that includes less frequent water changes than other approaches tend to use. Even so my hard water in the tanks is always sweet and clean because of the cleaning the plants do for me. I am sitting looking at my red tailed goodeid tank, a 55 gallon, and it has water lettuce across the whole water surface with roots extending almost down to the substrate. That is what I consider a large plant population. The stem plants, Vallisneria and Anubias are struggling a bit because of the shade cast by the water lettuce even though the tank runs at 2 WPG.