how do you heal the plants, theyre health is going down and watering doesnt halp nor the plant food

What it sounds like is that you have a fragile hybrid seed in normal potting soil. The trick is to sell enough "common" seed plants to be able to buy the advanced soil. THEN you can grow just about anything you want. There is an instant plant food "radio active plant food" that will restore to full health, but it gets mighty expensive to keep giving that to one plant. Save and get the high end soil, and that should solve your problems.