Don't worry, you are doing fine! I am a gardener and my motto is always give up on ugly plants and move on!
Trim plants heavily, ugly leaves never improve. As soon as you prune the plant wakes up and sends out new growth so make it a habit to attack dead and rotting plants daily!
The exceptions are young plants, they go through shock when you plant them so give them a few weeks to get settled, then prune lightly.
People will recommend fertilizers up the wazoo. Great. I use them too, but I learned how to garden by observing what thrives on my deck in my climate and what struggles, and planting accordingly.
In my aquarium I bought a round of plants and half took off, half just lived, and dwarf hair grass looked crappy. I pulled it out and put it in the graveyard, which is a cup of water on the window sill. My regular garden plants get sent to the graveyard as soon as they are done blooming or if they are the least awesome growers. Some survive, some die out. I never cry about it.
Let the plants decide if they want to live, thrive or die, remove your attachments to the outcome. Gardening is a joy!
